leetcode-865-Smallest Subtree with all the Deepest Nodes

With pattern:

if(node == NULL){
 // do something
}
left = do_something(root->left)
right = do_something(root->right)
do_something with root, left, right

we can get that, left and right return its depth and the deepest node’s parents

Error:

1. do not know the meaning of function and its return, with function depth(root), we return current depth of subtree, and the deepest node in the pair structure.