【LeetCode-面试算法经典-Java实现】【107-Binary Tree Level Order Traversal II（二叉树层序遍历II）】

【107-Binary Tree Level Order Traversal II（二叉树层序遍历II）】

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【LeetCode-面试算法经典-Java实现】【所有题目目录索引】

原题

　　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,#,#,15,7},

    3   / \  9  20    /  \   15   7

　　return its bottom-up level order traversal as:

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

题目大意

　　给定一棵二叉树自底向下进行层序遍历。

解题思路

　　对树进行层序遍历，每层的结果放在结果链表的头部。

代码实现

public class TreeNode {    int val;    TreeNode left;    TreeNode right;    TreeNode(int x) { val = x; }}

算法实现类

import java.util.*;public class Solution {    public List<List<Integer>> levelOrderBottom(TreeNode root) {        List<List<Integer>> list = new LinkedList<>();        if (root == null) {            return list;        }        Deque<TreeNode> cur = new LinkedList<>();        Deque<TreeNode> nxt = new LinkedList<>();        Deque<TreeNode> exc = new LinkedList<>();        TreeNode tmp;        cur.add(root);        while (!cur.isEmpty()) {            List<Integer> layout = new ArrayList<>();            while (!cur.isEmpty()) {                tmp = cur.remove();                if (tmp.left != null) {                    nxt.add(tmp.left);                }                if (tmp.right != null) {                    nxt.add(tmp.right);                }                layout.add(tmp.val);            }            exc = cur;            cur = nxt;            nxt = exc;            list.add(0, layout);        }        return list;    }}

评测结果

　　点击图片，鼠标不释放，拖动一段位置，释放后在新的窗口中查看完整图片。

特别说明

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