class Solution {
private:
    int min(int a, int b)
    {
        if (a < b)
            return a;
        return b;
    }
    int countDepth(TreeNode *cur)
    {
        if (cur->left == NULL || cur->right == NULL)
        {
            if (cur->left != NULL)
                return countDepth(cur->left) + 1;
            if (cur->right != NULL)
                return countDepth(cur->right) + 1;
            return 1;
        }
        return min(countDepth(cur->left), countDepth(cur->right)) + 1;
    }
public:
    int minDepth(TreeNode* root) {
        if (root == NULL)
            return 0;
        return countDepth(root);
    }
};