剑指offer04:重建二叉树

题目描述

输入某二叉树的前序遍历和中序遍历的结果，请重建出该二叉树。假设输入的前序遍历和中序遍历的结果中都不含重复的数字。例如输入前序遍历序列{1,2,4,7,3,5,6,8}和中序遍历序列{4,7,2,1,5,3,8,6}，则重建二叉树并返回。


 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     TreeNode* reConstructBinaryTree(vector<int> pre,vector<int> vin) {
13         unordered_map<int,int> index;
14         for(int i=0;i<vin.size();++i){
15             index[vin[i]] = i;
16         }
17         return buildtree(pre,vin,0,pre.size()-1,0,vin.size()-1,index);
18     }
19     
20     TreeNode* buildtree(
21         vector<int>&pre,
22         vector<int>&vin,
23         int pstart,
24         int pend,
25         int vstart,
26         int vend,
27         unordered_map<int,int>&index)
28     {
29         if(pstart > pend)return nullptr;
30         TreeNode *tempnode = new TreeNode(pre[pstart]);
31         int idx = index[pre[pstart]];
32         tempnode->left = buildtree(pre,vin,pstart+1,pstart+idx-vstart,vstart,idx-1,index);
33         tempnode->right = buildtree(pre,vin,pend-vend+idx+1,pend,idx+1,vend,index);
34         return tempnode;
35     }
36 };