LeetCode: 111. Minimum Depth of Binary Tree

题目

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

Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

Note: A leaf is a node with no children.

Example 1:

image

Input: root = [3,9,20,null,null,15,7]
Output: 2

Example 2:

Input: root = [2,null,3,null,4,null,5,null,6]
Output: 5

Constraints:

  • The number of nodes in the tree is in the range [0, 105].
  • -1000 <= Node.val <= 1000

解法

广度优先遍历，找到最近的那个没有子节点的节点，这个节点距离根节点的深度即为最小深度。

这个方法的 Python 代码类似下面这样:

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def minDepth(self, root: TreeNode) -> int:
        if root is None:
            return 0
        stack = []
        stack.append((root, 1))

        while stack:
            node, depth = stack.pop(0)
            if node.left is None and node.right is None:
                return depth

            if node.left is not None:
                stack.append((node.left, depth + 1))
            if node.right is not None:
                stack.append((node.right, depth + 1))

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