[leetcode][tree][dfs] 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.

/**
* 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 (NULL == root) return 0;
if (NULL == root->left) return 1 + minDepth(root->right);//！！！注意：要求的是根节点到叶节点（左右子树都为空）的距离
if (NULL == root->right) return 1 + minDepth(root->left);
return 1 + min(minDepth(root->left), minDepth(root->right));
}
};