103.Binary Tree Zigzag Level Order Traversal
===

###### tags: `Medium`,`Tree`,`BFS`,`Binary Tree`

[103. Binary Tree Zigzag Level Order Traversal](https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/)

### 題目描述

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).

### 範例

**Example 1:**


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

**Example 2:**

Input: root = [1]
Output: [[1]]

**Example 3:**

Input: root = []
Output: []

**Constraints**:

* The number of nodes in the tree is in the range `[0, 2000]`.
* -100 <= `Node.val` <= 100

### 解答
#### Javascript

function zigzagLevelOrder(root) {
  if (root === null) return [];
  const result = [];
  const queue = [root];
  let level = 0;
  while (queue.length) {
    const nodes = [];
    const size = queue.length;
    for (let i = 0; i < size; i++) {
      const node = queue.shift();
      if (level % 2 === 0) {
        nodes.push(node.val);
      } else {
        nodes.unshift(node.val);
      }

      if (node.left) queue.push(node.left);
      if (node.right) queue.push(node.right);
    }

    result.push(nodes);
    level++;
  }

  return result;
}

> [name=Marsgoat][time=Feb 20, 2023]

### Reference

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