Leetcode之Minimum Depth of Binary Tree

题目：

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.

代码：

/**
 * 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)return 0;
        if(!root->left&&!root->right)return 1;
        if(!root->left)return minDepth(root->right)+1;
        if(!root->right)return minDepth(root->left)+1;
        int l=minDepth(root->left);
        int r=minDepth(root->right);
        
        return min(l,r)+1;
    }
};

想法：

理清题意，最短路径应该是根节点到一个叶节点的最小的距离