some minor change to the recursive solution of the maximum depth problem

```
class Solution {
public:
int minDepth(TreeNode *root) {
if (!root) return 0;
if (!root->left) return 1 + minDepth(root->right);
else if (!root->right) return 1 + minDepth(root->left);
return 1 + min(minDepth(root->left), minDepth(root->right));
}
};
```