题目描述

从上到下按层打印二叉树，同一层的节点按从左到右的顺序打印，每一层打印到一行。

例如: 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],
  [9,20],
  [15,7]
]

思路

记录栈中节点元素的个数，作为当前层的节点个数，用来变量每一层的节点

代码实现

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

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */

function TreeNode(val) {
  this.val = val;
  this.left = this.right = null;
}

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];
    }
  }
}
console.log(arr);
/**
 *
 *
 * @param {TreeNode} root
 * @return {number[][]}
 */

var levelOrder = function (root) {
  if (!root) {
    return [];
  }
  // 记录下一层结点的个数
  let res = [];
  let levelRes = [];
  let nodes = [];
  nodes.push(root);
  let cnt = 1;
  let nextCnt = 0;
  while (nodes.length > 0) {
    /*         while(nodes.length > 0 && cnt > 0){
          // 放入层结果
          let cur = nodes.shift();
          levelRes.push(cur.val);
          if(cur.left){
              nodes.push(cur.left);
              nextCnt ++;
          }
          if(cur.right){
              nodes.push(cur.right);
              nextCnt ++;
          }
          cnt --;
      }
      // 放入最终结果
      res.push(levelRes);
      levelRes = [];
      cnt = nextCnt;
      nextCnt = 0;  */

    let len = nodes.length;
    let level = [];

    for (let i = 0; i < len; i++) {
      let cur = nodes.shift();
      // if (cur.val !== null) {
      //   level.push(cur.val);
      // }
      level.push(cur.val);
      if (cur.left) {
        nodes.push(cur.left);
        nextCnt++;
      }
      if (cur.right) {
        nodes.push(cur.right);
        nextCnt++;
      }
    }
    res.push(level);
  }

  return res;
};

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