leetcode107. Binary Tree Level Order Traversal II

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这里写图片描述 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   7return its bottom-up level order traversal as:[  [15,7],  [9,20],  [3]]

解法

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public List<List<Integer>> levelOrderBottom(TreeNode root) {        List<List<Integer>> ret = new ArrayList<List<Integer>>();        if (root == null) {            return ret;        }        Queue<TreeNode> queue = new LinkedList<TreeNode>();        queue.offer(root);        while(!queue.isEmpty()) {            List<Integer> level = new ArrayList<Integer>();            int size = queue.size();            for (int i = 0; i < size; i++) {                TreeNode head = queue.poll();                level.add(head.val);                if (head.left != null) {                    queue.offer(head.left);                }                if (head.right != null) {                    queue.offer(head.right);                }            }            ret.add(level);        }        Collections.reverse(ret);        return ret;    }}

这里写图片描述

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