README.md

Binary tree, a node is visible only if its value is the largest one on the path from root to itself, find all the visible nodes.

            2
         /     \
        7       1
       / \     / \
      6   5   3   4
visible nodes: [2,7,3,4]

Because we want the information from the root to each node, we can use backtracking. The information we pass down determines if current node is visibe. That is if the current cur value is larger than the max value from root to the path of current node except itself.
time: O(n) - n is the number of nodes in the tree.
space: O(height)

We cannot use pure recursion, because we cannot use the have the information of the max information of the path from root to the current node. When we use pure recursion we can only return the information from the smaller problem which is the subtrees up to the root.