难度系数：easy

/**

 * 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;

        if(root->left!=NULL&&root->right!=NULL)

            return min(minDepth(root->left)+1,minDepth(root->right)+1);

        else if(root->left==NULL&&root->right!=NULL) return minDepth(root->right)+1;

        else if(root->right==NULL&&root->left!=NULL) return minDepth(root->left)+1;

        else return 1;

    }

};