107. Binary Tree Level Order Traversal II

Problem:

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

For example: Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

Solution:

See 102. Binary Tree Level Order Traversal.

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
let levelOrderBottom = function (root) {
  if (!root) {
    return [];
  }

  const result = [];
  const queue = [NaN, root];
  while (queue.length > 1) {
    const node = queue.shift();
    if (node !== node) {
      result.unshift(queue.map((n) => n.val));
      queue.push(NaN);
    } else {
      if (node.left) {
        queue.push(node.left);
      }
      if (node.right) {
        queue.push(node.right);
      }
    }
  }

  return result;
};

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