#107 - Binary Tree Level Order Traversal II

Medium • View on LeetCode • View on GitHub

Problem Description

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).

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) return [];

  const result = [];
  const stack = [root];

  while (stack.length > 0) {
    const level = [];
    const n = stack.length;

    for (let i = 0; i < n; i++) {
      const node = stack.shift();
      level.push(node.val);

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

    result.unshift(level);
  }

  return result;
};