Minimum Depth of Binary Tree

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
 /*
    求二叉树的最小深度
 */
class Solution {
public:
    int res = INT_MAX;
    void dfs(TreeNode *root,int depth){
        if(!root||depth>=res) return;//剪枝，当前搜索深度大于最小值就不要继续往下搜了
        if(!root->left &&!root->right){
            res = min(res,depth+1);
            return ;
        }
        dfs(root->left,depth+1);
        dfs(root->right,depth+1);
        return ;
    }
    int minDepth(TreeNode *root) {
        if(!root) return 0;
        dfs(root,0);
        return res;
    }
};