BFS & DFS are the most common interview questions, they can solve one same questions with both solution. Let's go!
we trained the same question both with DFS & BFS!

MATRIX LEVEL -- BFS & DFS

733. Flood Fill

1) BFS

• Bullet Input: image = [[1,1,1],[1,1,0],[1,0,1]], sr = 1, sc = 1, newColor = 2
• Bullet Output: [[2,2,2],[2,2,0],[2,0,1]]
• Bullet Explanation: From the center of the image with position (sr, sc) = (1, 1) (i.e., the red pixel), all pixels connected by a path of the same color as the starting pixel (i.e., the blue pixels) are colored with the new color.
  Note the bottom corner is not colored 2, because it is not 4-directionally connected to the starting pixel.

Method 1: Do the task first then insert into queue!

import collections
def floodFill(self, image: List[List[int]], sr: int, sc: int, newColor: int) -> List[List[int]]:
    if image[sr][sc] == newColor:
        return image

    rows, cols = len(image), len(image[0])
    oldColor = image[sr][sc]

    # first do the task! then insert base element
    image[sr][sc] = newColor
    q = deque()
    q.append([sr,sc])

    # loop until q cannot go further
    while q:
        i, j = q.popleft()

        # find all possible direction for this point
        for x, y in ((i - 1, j), (i + 1, j), (i, j - 1), (i, j + 1)):
            if 0 <= x < rows and 0 <= y < cols and image[x][y] == oldColor:
                # first do! then insert surronding elements
                image[x][y] = newColor
                q.append([x, y])
    return image

Method 2: insert into queue then do stuff!

import collections
def floodFill(self, image: List[List[int]], sr: int, sc: int, newColor: int) -> List[List[int]]:
    if image[sr][sc] == newColor:
        return image

    rows, cols = len(image), len(image[0])
    oldColor = image[sr][sc]

    # insert base element first
    q = deque()
    q.append([sr,sc])

    # loop until q cannot go further
    while q:
        i, j = q.popleft()

        # do it here!
        image[i][j] = newColor

        # find all possible direction for this point
        for x, y in ((i - 1, j), (i + 1, j), (i, j - 1), (i, j + 1)):
            if 0 <= x < rows and 0 <= y < cols and image[x][y] == oldColor:
                # insert surrounding elements first!
                q.append([x, y])
    return image

2) DFS

matrix DFS ALWAYS use recursion, tree DFS can use DFS recursion or stack!

def floodFill(self, image: List[List[int]], sr: int, sc: int, newColor: int) -> List[List[int]]:
        # DFS ALWAYS start with a list of judgements for simplification
        if image[sr][sc] == newColor:
            return image

        rows, cols = len(image), len(image[0])

        # base and "always" operation
        oldColor = image[sr][sc]
        image[sr][sc] = newColor

        # recursion around all surronding (REMEBER To set up stop condition)
        for x, y in (sr, sc + 1), (sr + 1, sc), (sr - 1, sc), (sr, sc - 1):
            if 0 <= x < rows and 0 <= y < cols and image[x][y] == oldColor:
                self.floodFill(image, x, y, newColor)

        return image

200. Number of Islands

1) DFS

import collections
def numIslands(self, grid: List[List[str]]) -> int:

    rows = len(grid)
    cols = len(grid[0])
    count = 0

    def bfs(grid, m, n):
        res = 0
        if grid == None: return res

        q = deque()
        q.append((m, n))
        grid[m][n] = "#"

        while q:
            curNode = q.popleft()

            for x, y in (curNode[0] + 1, curNode[1]), (curNode[0], curNode[1] + 1), (curNode[0] - 1, curNode[1]), (curNode[0], curNode[1] - 1):
                if 0 <= x < rows and 0 <= y < cols and grid[x][y] == "1":
                    grid[x][y] = "#"
                    q.append((x, y))

    for row in range(rows):
        for col in range(cols):
            if grid[row][col] == "1":
                bfs(grid, row, col)
                count += 1

    return count

2) BFS

def numIslands(self, grid: List[List[str]]) -> int:

        def dfs(grid, i, j):
            if i<0 or j<0 or i>=len(grid) or j>=len(grid[0]) or grid[i][j] != '1':
                return
            grid[i][j] = '#'  # THE MOST IMPORTANT STEP, VISITED STEP
            dfs(grid, i+1, j)
            dfs(grid, i-1, j)
            dfs(grid, i, j+1)
            dfs(grid, i, j-1)

        if not grid:
            return 0

        count = 0
        for i in range(len(grid)):
            for j in range(len(grid[0])):
                if grid[i][j] == '1':
                    dfs(grid, i, j)
                    count += 1
        return count

490. The Maze

https://assets.leetcode.com/uploads/2021/03/31/maze1-1-grid.jpg

Input: maze = [[0,0,1,0,0],[0,0,0,0,0],[0,0,0,1,0],[1,1,0,1,1],[0,0,0,0,0]], start = [0,4], destination = [4,4]
Output: true
Explanation: One possible way is : left -> down -> left -> down -> right -> down -> right.

1) BFS

import collections
    import collections
    def hasPath(self, maze: List[List[int]], start: List[int], destination: List[int]) -> bool:
        rows = len(maze)
        cols = len(maze[0])
        dir = [1, 0, -1, 0, 1]
        visited = [[0 for _ in range(cols)] for _ in range(rows)]
        q = deque()
        q.append(start)
        visited[start[0]][start[1]] = 1

        while q: 
            cur = q.popleft()
            if cur[0] == destination[0] and cur[1] == destination[1]:
                return True

            for i in range(4):
                newX = cur[0]
                newY = cur[1]
                while 0 <= newX < rows and 0 <= newY < cols and maze[newX][newY] != 1:
                    newX += dir[i]
                    newY += dir[i + 1]
                newX -= dir[i]
                newY -= dir[i + 1]

                if visited[newX][newY]: continue
                q.append([newX, newY])
                visited[newX][newY] = 1

        return False

505. The Maze II

https://assets.leetcode.com/uploads/2021/03/31/maze1-1-grid.jpg

• Bullet Input: maze = [[0,0,1,0,0],[0,0,0,0,0],[0,0,0,1,0],[1,1,0,1,1],[0,0,0,0,0]], start = [0,4], destination = [4,4]
• Bullet Output: 12
• Bullet Explanation: One possible way is : left -> down -> left -> down -> right -> down -> right.
  The length of the path is 1 + 1 + 3 + 1 + 2 + 2 + 2 = 12.

1) BFS

    import collections
    def shortestDistance(self, maze: List[List[int]], start: List[int], destination: List[int]) -> int:
        rows = len(maze)
        cols = len(maze[0])

        dir = [1, 0, -1, 0, 1]
        res = [[float('inf') for _ in range(cols)] for _ in range(rows)]
        q = deque()
        q.append([start[0], start[1], 0])

        while q: 
            cur = q.popleft()
            if cur[2] >= res[cur[0]][cur[1]]:
                continue
            res[cur[0]][cur[1]] = cur[2]

            for i in range(4):
                newX = cur[0]
                newY = cur[1]
                path = cur[2]
                while 0 <= newX < rows and 0 <= newY < cols and maze[newX][newY] == 0:
                    newX += dir[i]
                    newY += dir[i + 1]
                    path += 1
                newX -= dir[i]
                newY -= dir[i + 1]
                path -= 1
                q.append([newX, newY, path])

        return -1 if res[destination[0]][destination[1]] == float('inf') else res[destination[0]][destination[1]]

ARRAY LEVEL -- BFS & DFS

78. Subsets

Solution 1

Input: nums = [1,2,3]
Output: [[],[1],[2],[1,2],[3],[1,3],[2,3],[1,2,3]]

def subsets(self, nums: List[int]) -> List[List[int]]:
    def backtract(sublist, templist, nums, start):
        sublist.append(templist[:])
        for i in range(start, len(nums)):
            templist.append(nums[i])
            backtract(sublist, templist, nums, i + 1)
            templist.pop()
        return sublist

    sublist = []
    nums.sort()
    return backtract(sublist, [], nums, 0)

Solution 2

def subsets(self, nums):
    result = [[]]
    for num in nums:
        result += [i + [num] for i in result]
    return result

90. Subsets II

Input: nums = [1,2,2]
Output: [[],[1],[1,2],[1,2,2],[2],[2,2]]

def subsetsWithDup(self, nums: List[int]) -> List[List[int]]:
    def backtract(sublist, templist, nums, start):
        sublist.append(templist[:])
        for i in range(start, len(nums)):
            if i > start and nums[i] == nums[i - 1]:
                continue
            templist.append(nums[i])
            backtract(sublist, templist, nums, i + 1)
            templist.pop()
        return sublist

    sublist = []
    nums.sort()
    return backtract(sublist, [], nums, 0)

46. Permutations

Input: nums = [1,2,3]
Output: [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]

def backtract(self, reslist, templist, nums) -> List[List[int]]:
        if len(templist) == len(nums):
            reslist.append(templist[:])
        else:
            for i in range(len(nums)):
                if nums[i] in templist:
                    continue
                templist.append(nums[i])
                self.backtract(reslist, templist, nums)
                templist.pop()
        return reslist

def permute(self, nums: List[int]) -> List[List[int]]:
    reslist = []
    nums.sort()
    return self.backtract(reslist, [], nums)

47. Permutations II

Input: nums = [1,1,2]
Output:
[[1,1,2],
[1,2,1],
[2,1,1]]

def backtract(self, reslist, templist, nums, used) -> List[List[int]]:
        if len(templist) == len(nums):
            reslist.append(templist[:])
        else:
            for i in range(len(nums)):
                if used[i] == 1 or i > 0 and nums[i] == nums[i - 1] and not used[i - 1]:
                    continue
                used[i] = 1
                templist.append(nums[i])
                self.backtract(reslist, templist, nums, used)
                used[i] = 0
                templist.pop()
        return reslist

def permuteUnique(self, nums: List[int]) -> List[List[int]]:
    reslist = []
    boolean = [0 for _ in range(len(nums))]
    nums.sort()
    return self.backtract(reslist, [], nums, boolean)

39. Combination Sum

Input: candidates = [2,3,6,7], target = 7
Output: [[2,2,3],[7]]
Explanation:
2 and 3 are candidates, and 2 + 2 + 3 = 7. Note that 2 can be used multiple times.
7 is a candidate, and 7 = 7.
These are the only two combinations.

def backtrack(self, reslist, templist, nums, remain, start) -> List[List[int]]:
        if remain < 0: 
            return
        elif remain == 0:
            reslist.append(templist[:])
        else:
            for i in range(start, len(nums)):
                templist.append(nums[i])
                self.backtrack(reslist, templist, nums, remain - nums[i], i)
                templist.pop()
        return reslist

    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
        reslist = []
        candidates.sort()
        return self.backtrack(reslist, [], candidates, target, 0)

40. Combination Sum II

Input: candidates = [10,1,2,7,6,1,5], target = 8
Output:
[
[1,1,6],
[1,2,5],
[1,7],
[2,6]
]

def backtrack(self, reslist, templist, nums, remain, start) -> List[List[int]]:
        if remain < 0: 
            return
        elif remain == 0:
            reslist.append(templist[:])
        else:
            for i in range(start, len(nums)):
                if i > start and nums[i] == nums[i - 1]:
                    continue
                templist.append(nums[i])
                self.backtrack(reslist, templist, nums, remain - nums[i], i + 1)
                templist.pop()
        return reslist

def combinationSum2(self, candidates: List[int], target: int) -> List[List[int]]:
    reslist = []
    candidates.sort()
    return self.backtrack(reslist, [], candidates, target, 0)

131. Palindrome Partitioning

Input: s = "aab"
Output: [["a","a","b"],["aa","b"]]

https://leetcode.com/problems/palindrome-partitioning/Figures/131/time_complexity.png

    def isPalindrome(self, s, start, end) -> bool:
        temp = s[start:end + 1]
        return temp == temp[::-1]

    def backtrack(self, reslist, templist, s, start) -> List[List[int]]:
        if start == len(s):
            reslist.append(templist[:])
        else:
            for i in range(start, len(s)):
                if self.isPalindrome(s, start, i):
                    templist.append(s[start: i + 1])
                    self.backtrack(reslist, templist, s, i + 1)
                    templist.pop()

        return reslist

    def partition(self, s: str) -> List[List[str]]:
        reslist = []
        return self.backtrack(reslist, [], s, 0)

102. Binary Tree Level Order Traversal

Why this is wrong?

def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
    import collections
    if root == None: return res

    res = []
    q = deque()
    q.append(root)

    while q:
        temp = q.popleft()
        res.append(temp.val)

        if temp.left != None:
            q.append(temp.left)

        if temp.right != None:
            q.append(temp.right)

    return res

Use your level

    import collections
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:

        res = []
        if root == None: return res

        q = deque()
        q.append(root)

        while q:

            # SIZE! OF! THE! QUEUE!
            size = len(q)

            curLevel = []
            for i in range(size):
                curNode = q.popleft()
                curLevel.append(curNode.val)

                if curNode.left != None:
                    q.append(curNode.left)

                if curNode.right != None:
                    q.append(curNode.right)

            res.append(curLevel)
        return res

BINARY TREE LEVEL -- BFS & DFS

103. Binary Tree Zigzag Level Order Traversal

     
     import collections
     def zigzagLevelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
     res = []
     
     if root == None: return res
     
     q = deque()
     q.append(root)
     
     counter = 0
     while q:
         curLevel = []
         for i in range(len(q)):
             curNode = q.popleft()
             curLevel.append(curNode.val)
     
             if curNode.left:
                 q.append(curNode.left)
     
             if curNode.right:
                 q.append(curNode.right)
     
         if counter % 2 == 0:
             res.append(curLevel)
         else:
             res.append(curLevel[::-1])
     
         counter += 1
     
     return res

98. Validate Binary Search Tree

Template of inorder traversal
```python
def isValidBST(self, root: Optional[TreeNode]) -> bool:
    if root == None: return True
    stack = []
    ls = []

    while stack or root:
        while root:
            stack.append(root)
            root = root.left

        root = stack.pop()

        # condition here
        root = stack.pop()
        ls.append(root.val)

        root = root.right

    return ls</code></pre>
<p>Inorder traversal</p>
<pre><code class="language-python">class Solution:
    def isValidBST(self, root: Optional[TreeNode]) -> bool:
        if root == None: return True

        stack = []
        pre = None

        while stack or root:
            while root:
                stack.append(root)
                root = root.left

            root = stack.pop()

            # greater or equal than!
            if pre and pre.val >= root.val:
                return False

            pre = root
            root = root.right

        return True</code></pre>
<p>dfs</p>
<pre><code class="language-python">
    def isValidBST(self, root: Optional[TreeNode]) -> bool:
        def isValidBST(root, minValue, maxValue):
            if root == None: return True

            if root.val >= maxValue or root.val <= minValue:
                return False

            return isValidBST(root.left, minValue, root.val) and isValidBST(root.right, root.val, maxValue)

        return isValidBST(root, -sys.maxsize, sys.maxsize)  </code></pre>
<ol start="230">
<li>
<p>Kth Smallest Element in a BST</p>
<pre><code class="language-python">
def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
if root == None: return -1

stack = []

while root or stack:
    while root:
        stack.append(root)
        root = root.left

    root = stack.pop()

    if k == 1: return root.val
    k -= 1

    root = root.right

return -1</code></pre>
</li>
</ol>
<pre><code>
114. Flatten Binary Tree to Linked List
```python
def flatten(self, root: Optional[TreeNode]) -> None:
        def dfs(root):
            if root == None: return None
            leftLast = dfs(root.left)
            rightLast = dfs(root.right)

            if leftLast:

                ############
                leftLast.right = root.right
                root.right = root.left
                root.left = None
                ############

            if rightLast != None:
                return rightLast
            if leftLast != None:
                return leftLast

            return root

        dfs(root)

199. Binary Tree Right Side View

Why this is wrong?

def rightSideView(self, root: Optional[TreeNode]) -> List[int]:
    res = []
    if root == None: return res

    q = collections.deque()
    q.append(root)
    res.append(root.val)
    while q:
        for i in range(len(q)):
            curNode = q.popleft()

            if i == len(q) - 1:
                res.append(curNode.val)
            if curNode.left:
                q.append(curNode.left)
            if curNode.right:
                q.append(curNode.right)

    return res

It should be:

def rightSideView(self, root: Optional[TreeNode]) -> List[int]:
    res = []
    if root == None: return res

    q = collections.deque()
    q.append(root)
    while q:
        size = len(q)
        for i in range(size):
            curNode = q.popleft()

            if curNode.left:
                q.append(curNode.left)

            if curNode.right:
                q.append(curNode.right)

            if i == size - 1:
                res.append(curNode.val)
    return res

dfs

def rightSideView(self, root: Optional[TreeNode]) -> List[int]:
    def dfs(root, depth, res):
        if root == None: return

        if depth == len(res):
            res.append(root.val)

        dfs(root.right, depth + 1, res)
        dfs(root.left, depth + 1, res)

    res = []
    dfs(root, 0, res)
    return res