103. Binary Tree Zigzag Level Order Traversal（js）

103. Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / 
  9  20
    /  
   15   7

return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]
]
题意：二叉搜索树的层序遍历的变形，z形遍历
代码如下：

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var zigzagLevelOrder = function(root) {
    let res=[];
   helper(root,res,0);
    return res;
};

var helper=function(root,res,level){
    if(!root) return ;
    if(!res[level]) res[level]=[];
    level%2 ? res[level].unshift(root.val) :res[level].push(root.val);
    helper(root.left,res,level+1);
    helper(root.right,res,level+1);
}