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Binary Tree Zigzag Level Order Traversal

Problem statement​

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 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

My solution​

/**
 * 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[][]}
 */
var zigzagLevelOrder = function(root) {
    if (!root) {
        return [];
    }
    
    const result = [];
    const queue = [root];
    let level = 0;
    
    while (queue.length) {
        const size = queue.length;
        const localResult = []
        
        for (let i = 0; i < size; i++) {
            const head = queue.shift();
            
            if (level % 2 === 0) {
                localResult.push(head.val)
            } else {
                localResult.unshift(head.val)
            }
            
            if (head.left) {
                queue.push(head.left)
            }
            
            if (head.right) {
                queue.push(head.right)
            }
        }
        
        result.push(localResult)
        level++
        
    }
    
    return result
};