Binary Tree Right Side View

Description

Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

Example:

Input: [1,2,3,null,5,null,4]
Output: [1, 3, 4]
Explanation:

   1            <---
 /   \
2     3         <---
 \     \
  5     4       <---

Solution(javascript)

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
const rightSideView = (root) => {
  let frontier = [root]
  const res =  []
  while (frontier.length) {
    if(frontier[frontier.length - 1]) {
        res.push(frontier[frontier.length - 1].val)   
    }

    frontier = frontier.reduce((next, node) => {
        if(!node) {
            return next
        }
      if (node.left) {
        next.push(node.left)
      }
      if (node.right) {
        next.push(node.right)
      }
      return next
    }, [])
  }
  return res
}