leetcode 111. Minimum Depth of Binary Tree

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.

/**
 * 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;
        }
        else if(root->left == NULL) {
            return minDepth(root->right) + 1;
        }
        else if(root->right == NULL) {
            return minDepth(root->left) + 1;
        }
        else {
            return min(minDepth(root->left), minDepth(root->right)) + 1;
        }
    }
};