leetcode-java-107. Binary Tree Level Order Traversal II

/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ /* 先层次遍历出list 再让list倒置 */ public class Solution { public List<List<Integer>> levelOrderBottom(TreeNode root) { List<List<Integer>> list = new LinkedList<List<Integer>>(); Queue<TreeNode> queue = new LinkedList<TreeNode>(); if(root == null) { return list; } queue.offer(root); while(!queue.isEmpty()) { int m = queue.size(); List<Integer> level = new LinkedList<Integer>(); while(m > 0) { TreeNode node = queue.peek(); if(node.left != null) { queue.offer(node.left); } if(node.right != null) { queue.offer(node.right); } level.add(queue.poll().val); m--; } list.add(level); } Collections.reverse(list); return list; } }