[LeetCode] 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) {}
 * };
 */
class Solution {
public:
    int min = 2147483647;
    void calc(int depth, TreeNode *node)
    {
        if(node == NULL) return;
        if(node -> left == NULL && node -> right == NULL)
        {
            if(depth < min)
                min = depth + 1;
        }
        else
        {
            if(node -> left != NULL)
                calc(depth + 1, node -> left);
            if(node -> right != NULL)
                calc(depth + 1, node -> right);
        }
    }
    
    int minDepth(TreeNode *root) {
        if(root == NULL) return 0;
        calc(0, root);
        return min;
    }
};