每日两题 T31

输入: [1,2,3,null,5,null,4]
输出: [1, 3, 4]
解释:

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

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var rightSideView = function(root) {
  if(!root) return []
  let arr = []
  dfs(root, 0, arr)
  return arr
};
function dfs (root, step, res) {
  if(root){
    if(res.length === step){
      res.push(root.val)           // 当数组长度等于当前 深度 时, 把当前的值加入数组
    }
    dfs(root.right, step + 1, res) // 先从右边开始, 当右边没了, 再轮到左边
    dfs(root.left, step + 1, res)
  }
}

![1,2,3].includes(+a)

!['1', '2', '3'].includes(a + '')

!{1: true, 2: true, 3: true}[a]