107. Binary Tree Level Order Traversal II

107. Binary Tree Level Order Traversal II

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],

\ 9 20 / \

return its bottom-up level order traversal as:

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

/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */class Solution { public List> levelOrderBottom(TreeNode root) { List> traversal = new ArrayList>(); if(root == null) return traversal; LinkedList queue = new LinkedList(); queue.add(root); while(!queue.isEmpty()) { int levelLen = queue.size(); List levelNodes = new ArrayList(); for (int i = 0; i < levelLen; i++) { TreeNode node = queue.removeFirst(); levelNodes.add(node.val); if(node.left != null) queue.add(node.left); if(node.right != null) queue.add(node.right); } traversal.add(0, levelNodes); } return

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