题目描述

请实现一个函数按照之字形打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右至左的顺序打印，第三行按照从左到右的顺序打印，其他行以此类推。

测试用例

给定二叉树: [3,9,20,null,null,15,7]

   3
   / \
  9  20
    /  \
   15   7

返回其层次遍历结果：

[
  [3],
  [20,9],
  [15,7]
]

代码实现

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var levelOrder = function (root) {
  if (!root) {
    return [];
  }
  let nodes = [];
  nodes.push(root);
  let res = [];
  // true表示从左到右
  // false表示从右到左
  let flag = true;

  while (nodes.length) {
    let temp = [];
    let len = nodes.length;
    for (let i = 0; i < len; i++) {
      let cur = nodes.shift();
      flag === true ? temp.push(cur.val) : temp.unshift(cur.val);
      if (cur.left) {
        nodes.push(cur.left);
      }
      if (cur.right) {
        nodes.push(cur.right);
      }
    }
    res.push(temp);
    flag = !flag;
  }
  return res;
};

function TreeNode(val, left, right) {
  this.val = val === undefined ? 0 : val;
  this.left = left === undefined ? null : left;
  this.right = right === undefined ? null : right;
}

var __readline = require("readline-sync");
__readline.setDefaultOptions({ prompt: "" });
var readline = __readline.prompt;

console.log("请输入二叉树数组");
const a = readline()
  .split(",")
  .map((item) => {
    if (item == "null") {
      return null;
    } else {
      return Number(item);
    }
  });

let arr = [];
// 二叉树数组内的值转为TreeNode
for (let i = 0; i < a.length; i++) {
  arr[i] = new TreeNode(a[i]);
}

// 构造二叉树
for (let i = 0; i <= arr.length / 2 + 1; i++) {
  if (arr[2 * i + 1]) {
    if (arr[2 * i + 1].val != null) {
      arr[i].left = arr[2 * i + 1];
    }
  }
  if (arr[2 * i + 2]) {
    if (arr[2 * i + 2].val != null) {
      arr[i].right = arr[2 * i + 2];
    }
  }
}

const res = levelOrder(arr[0]);
console.log("结果为: ", res);

/* 
  请输入二叉树数组
  3,9,20,null,null,15,7
  结果为:  [ [ 3 ], [ 20, 9 ], [ 15, 7 ] ]
*/