Minimum Depth of Binary Tree

/**
 * 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;
        dep(root,0);
    }
    
    int dep(TreeNode* root,int l){
        if(root==NULL) return INT_MAX;
        if(root -> left ==NULL && root->right ==NULL) return l+1;
        return min(dep(root->left,l+1),dep(root->right,l+1));
    }
};

注意题目要求，必须是到叶子结点。