二叉树层次遍历c语言_LeetCode 树 107.二叉树的层次遍历Ⅱ

5(107) 二叉树的层次遍历Ⅱ

描述

给定一个二叉树，返回其节点值自底向上的层次遍历。（即按从叶子节点所在层到根节点所在的层，逐层从左向右遍历）

示例

给定二叉树 [3,9,20,null,null,15,7]，

    3/ 9  20/  15   7

返回其自底向上的层次遍历为：

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

Description

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).

Example

Given binary tree [3,9,20,null,null,15,7]

    3/ 9  20/  15   7

return its bottom-up level order traversal as:

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

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解题

此题目是4.102.二叉树的层次遍历的一种变形即层次遍历后输出的数组是逆序的因此只是在102题目的代码中修改了输出数组的顺序很多题解都是在102题目的基础上在最后输出数组的时候使用了reverse(res.begin(),res.end())调换了数组内元素的顺序但本人的代码是使用了stack<vector<int>>来接收每次层序遍历输出的数组元素然后再使用一次while循环将stack栈顶的元素添加到二维数组res中

#include <iostream>
#include <vector>
#include <queue>
#include <stack>
using namespace std;
struct TreeNode{int val;TreeNode *left;TreeNode *right;TreeNode(int x):val(x),left(NULL),right(NULL) {}
};
TreeNode* levelCreateBinaryTree(const vector<int> &nums,int len,int index){//层序创建二叉树index为位置序号TreeNode *root=NULL;if(index<len&&nums[index]!=-1){root = new TreeNode(nums[index]);root->left = levelCreateBinaryTree(nums,len,2*index+1);root->right= levelCreateBinaryTree(nums,len,2*index+2);}return root;
}
void PrintMartrix(vector<vector<int>>& res){//打印二维数组for(int i=0;i<res.size();++i){cout<<"[";for(int j=0;j<res[i].size();++j){cout<<" "<<res[i][j]<<" ";}cout<<"]"<<endl;}
}
class Solution {public:vector<vector<int>> levelOrderBottom(TreeNode* root) {vector<vector<int>> res;stack<vector<int>> st;if(!root) return res;queue<TreeNode* > Tree;Tree.push(root);while(!Tree.empty()){vector<int> temp;int len=Tree.size();while(len--){TreeNode *pNode=Tree.front();Tree.pop();temp.push_back(pNode->val);if(pNode->left) Tree.push(pNode->left);if(pNode->right) Tree.push(pNode->right);}st.push(temp);}while(!st.empty()){res.push_back(st.top());st.pop();}return res;}
};
int main(){vector<int> nums={0,1,2,3,4,5,6,7,8};//示例 -1代表所在位置为空值int len=nums.size();TreeNode *root=levelCreateBinaryTree(nums,len,0);Solution answer;vector<vector<int>> res=answer.levelOrderBottom(root);PrintMartrix(res);return 0;
}