LeetCode 111. Minimum Depth of Binary Tree（easy）

题目链接：点击这里

注意单叉树的存在。

/**
 * 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 leftDepth = minDepth(root->left);
        int rightDepth = minDepth(root->right);
        if(leftDepth==0)    return rightDepth+1;
        if(rightDepth==0)   return leftDepth+1;
        return min(leftDepth, rightDepth) + 1;
    }
};