111. Minimum Depth of Binary Tree

https://leetcode.com/problems/minimum-depth-of-binary-tree/

Python

DFS Postfix

class Solution:
    def minDepth(self, root: Optional[TreeNode]) -> int:
        def dfs(node):
            if not node:
                return 0

            if node.right is None:
                return dfs(node.left) + 1
            if node.left is None:
                return dfs(node.right) + 1

            left, right = dfs(node.left), dfs(node.right)
            return min(left, right) + 1

        return dfs(root)

BFS

from collections import deque


class Solution:
    def minDepth(self, root: Optional[TreeNode]) -> int:
        if not root:
            return 0

        level, queue = 0, deque([root])

        while queue:
            level += 1

            for _ in range(len(queue)):
                node = queue.popleft()

                if not node.left and not node.right:
                    return level

                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)