338. Right Side View
Given a binary tree, output the right side view of the tree in top-to-bottom order. The right side view consists of the nodes that are visible when standing to the right of the tree.
The input consists of a level order traversal of the binary tree. Each node is represented by a value, and a -1 represents a null child.
Print the right side view as a space-separated list of integers in top-to-bottom order.
1 2 3 -1 -1 -1 -1
1<=number of nodes in the binary tree <=10^5