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;

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

        int leftDepth = minDepth(root->left);
        int rightDepth = minDepth(root->right);
        if(leftDepth > rightDepth)
            return rightDepth + 1;
        else
            return leftDepth + 1;
    }
};