111. Minimum Depth of Binary Tree(二叉树最小深度)

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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:

Given binary tree [3,9,20,null,null,15,7],

return its minimum depth = 2.

解法：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def minDepth(self, root: TreeNode) -> int:
        if not root:
            return 0
        def depth(node):
            if not node:
                return 0
            left=depth(node.left)
            right=depth(node.right)
            if left==0 or right==0:
                return left+right+1
            else:
                return min(left,right)+1
        return depth(root)

解释：

当左右子节点深度其中一个为0的时候，那么此时的深度由另外一边的深度+1来代替

if left==0 or right==0: return left+right+1

这个写法兼容两种情况，毕竟任何一个变量为0的时候就是相当于另外一个节点的深度+1

除此之外的情况就是当两个节点的深度都不为0的时候那么就是二者的最小值+1

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