【leetcode】【easy】111. Minimum Depth of Binary Tree

111. 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:

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

    3
   / \
  9  20
    /  \
   15   7

return its minimum depth = 2.

题目链接：https://leetcode-cn.com/problems/minimum-depth-of-binary-tree/

思路

递归思想的练习。非递归不写了。

相比求最大深度的题，本题多了一个小陷阱：深度代表的是从根节点出发到叶子节点结束。

特殊：[3,null,20,15,7]，答案是3二不是1。

因此向上返回长度时，需要判断当前节点是否左右都有孩子，有则返回左右中较小深度，无则返回较大深度（较小值其实就是0）。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int minDepth(TreeNode* root) {
        if(root==NULL) return 0;
        int l = minDepth(root->left);
        int r = minDepth(root->right);
        if(root->left && root->right) return min(l,r)+1;
        else return max(l,r)+1;
    }
};