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;
        int lh = minDepth(root->left);
        int rh = minDepth(root->right);
        if(lh==0 && rh==0){//左右子树都空
            return 1;
        }
        if(lh==0){//左子树空
            return rh+1;
        }
        if(rh==0){//右子树空
            return lh+1;
        }
        return lh<rh ? lh+1:rh+1;//左右子树都不空
    }
};