1.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.
2.题目要求：求二叉树的最小深度即最短路径的节点个数。
3.方法：用递归的方法，深度优先搜索DFS来完成。
4.代码：
/**
Definition for binary tree
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;
if (root->left == NULL && root->right == NULL) return 1;if (root->left == NULL) return minDepth(root->right) + 1; else if (root->right == NULL) return minDepth(root->left) + 1; else return 1 + min(minDepth(root->left), minDepth(root->right));}
};
