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Binary Tree Level Order Traversal II

Problem statement​

Given the root of a binary tree, return the bottom-up level order traversal of its nodes' values. (i.e., from left to right, level by level from leaf to root).

Example 1:

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

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].
  • -1000 <= Node.val <= 1000

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 levelOrderBottom = function(root) {
    if (root === null) {
        return []
    }
    const queue = [root]
    const results = [];
    
    while (queue.length) {
        const len = queue.length;
        const current = [];
        
        for (let i = 0; i < len; i++) {
            const head = queue.shift();
            
            current.push(head.val);
            
            if (head.left) {
                queue.push(head.left)
            }
            if (head.right) {
                queue.push(head.right)
            }
        }
        
        results.unshift(current)
    }
    
    return results
};