LeetCode-103. Binary Tree Zigzag Level Order Traversal

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问题描述

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

Example :

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Input: root = [3,9,20,null,null,15,7]
Output: [[3],[20,9],[15,7]]

解答

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/*
 * @Description: 103. Binary Tree Zigzag Level Order Traversal
 * @Author: libk
 * @Github: https://github.com/libk
 */
/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
const zigzagLevelOrder = function(root) {
  if (!root) {
    return []
  }

  let queue = [root]
  let res = []
  let leftToRight = true

  while (queue.length) {
    let curLength = queue.length
    let curLevel = []

    for (let i = 0; i < curLength; i++) {
      let cur = queue.shift()
      if (leftToRight) {
        curLevel.push(cur.val)
      } else {
        curLevel.unshift(cur.val)
      }
      if (cur.left) {
        queue.push(cur.left)
      }
      if (cur.right) {
        queue.push(cur.right)
      }
    }
    leftToRight = !leftToRight
    res.push(curLevel)
  }
  return res
}