【LeetCode】BinaryTreeLevelOrderTraversal_ii 二叉树的层次遍历 II

二叉树的层次遍历 II
给出一棵二叉树，返回其节点值从底向上的层次序遍历（按从叶节点所在层到根节点所在的层遍历，然后逐层从左往右遍历）

样例给出一棵二叉树 {3,9,20,#,#,15,7},    3   / \  9  20    /  \   15   7按照从下往上的层次遍历为：[  [15,7],  [9,20],  [3]]

标签
队列 二叉树 二叉树遍历 宽度优先搜索
相关题目
中等Search Graph Nodes45 %

（1）Java

import BinaryTree.TreeNode;import java.util.*;public class BinaryTreeLevelOrderTraversal_ii {    /*     * @param root: A tree     * @return: buttom-up level order a list of lists of integer     */    public List<List<Integer>> levelOrderBottom(TreeNode root) {        List<List<Integer>> results = new ArrayList<>();        if(root == null){            return results;        }        Queue<TreeNode> queue = new LinkedList<TreeNode>();        queue.offer(root);        while(!queue.isEmpty()){            List<Integer> buffer = new ArrayList<>();//must be defined in while-loop!!            int size = queue.size();            for(int i = 0; i < size; i++){                TreeNode head = queue.poll();                buffer.add(head.val);                if(head.left != null){                    queue.offer(head.left);                }                if(head.right != null){                    queue.offer(head.right);                }            }            results.add(buffer);        }        Collections.reverse(results);//        return results;    }}

（2）C++

#include <stdio.h>#include <vector>#include <queue>#include <algorithm>using namespace std;/*--- binary-tree-level-order-traversal(ii) ---*/// Definition of TreeNode:class TreeNode {public:    int val;    TreeNode *left, *right;    TreeNode(int val) {        this->val = val;        this->left = this->right = NULL;    }};//Version 1 : 1 Queue【Recommend】class BinaryTreeLevelOrderTraversal_ii{public:    /*     * @param root: A tree     * @return: buttom-up level order a list of lists of integer     */    vector<vector<int>> levelOrderBottom(TreeNode * root) {        // write your code here        vector<vector<int>> results;        if(root == NULL){            return results;        }        queue<TreeNode*> Q;        Q.push(root);        while (!Q.empty()) {            vector<int> buffer;            int size = Q.size();            for(int i = 0; i < size; i++){                TreeNode* root = Q.front(); Q.pop();                buffer.push_back(root->val);                if(root->left != NULL){                    Q.push(root->left);                }                if(root->right != NULL){                    Q.push(root->right);                }            }            results.push_back(buffer);        }        reverse(results.begin(), results.end());        return results;    }};//Version 2 : 1 Queue + Dummy Nodeclass BinaryTreeLevelOrderTraversal_ii_V2 {    /**     * @param root : The root of binary tree.     * @return : buttom-up level order a list of lists of integer     */public:    vector<vector<int>> levelOrderBottom(TreeNode *root) {        // write your code here        vector<vector<int> > ret;        ret.clear();        if(root == NULL)            return ret;        queue<TreeNode*> S;        S.push(root);        S.push(NULL);        vector<int> tmp;        while(!S.empty()){            //travesal current level            TreeNode* p = S.front();            S.pop();            if(p!=NULL)            {                tmp.push_back(p->val);                if(p->left)                    S.push(p->left);                if(p->right)                    S.push(p->right);            }else{                if(!tmp.empty())                {                    S.push(NULL);                    ret.push_back(tmp);                    tmp.clear();                }            }        }        reverse(ret.begin(),ret.end());        return ret;    }};