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.

int minDepth(TreeNode *root)     {        if (root == NULL)            return 0;                    int left_minimum = minDepth(root->left);        int right_minimum = minDepth(root->right);        if (left_minimum == 0 || right_minimum == 0)            return max(left_minimum, right_minimum) + 1;                    return min(left_minimum, right_minimum) + 1;            }