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 /**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* reConstructBinaryTree(vector<int> pre,vector<int> vin) {
        unordered_map<int,int> index;
        for(int i=0;i<vin.size();++i){
            index[vin[i]] = i;
        }
        return buildtree(pre,vin,0,pre.size()-1,0,vin.size()-1,index);
    }
    
    TreeNode* buildtree(
        vector<int>&pre,
        vector<int>&vin,
        int pstart,
        int pend,
        int vstart,
        int vend,
        unordered_map<int,int>&index)
    {
        if(pstart > pend)return nullptr;
        TreeNode *tempnode = new TreeNode(pre[pstart]);
        int idx = index[pre[pstart]];
        tempnode->left = buildtree(pre,vin,pstart+1,pstart+idx-vstart,vstart,idx-1,index);
        tempnode->right = buildtree(pre,vin,pend-vend+idx+1,pend,idx+1,vend,index);
        return tempnode;
    }
};