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.
思路：
利用递归的方法，当左右子节点都是NULL的时候能够判断当前节点是叶子节点。
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;
}
else if(root->left == NULL && root->right == NULL){
return 1;
}
else if(root->left == NULL){
return minDepth(root->right) + 1;
}else if(root->right == NULL){
return minDepth(root->left) + 1;
}
else{
int l = minDepth(root->left);
int r = minDepth(root->right);
return (l < r ? l : r) + 1;
}
}
};