剑指offer 4 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。

/**
 * 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) {
    TreeNode* root = helper(pre, 0, pre.size()-1, vin, 0, vin.size()-1);
        return root;
    }
    TreeNode* helper(vector<int> pre, int startpre, int endpre, vector<int> vin, int startvin, int endvin){
        if(startpre>endpre||startvin>endvin)
            return NULL;
        TreeNode* root=new TreeNode(pre[startpre]);
        for(int i=startvin;i<=endvin;i++){
            if(vin[i]==pre[startpre]){
                root->left = helper(pre, startpre+1, startpre+i-startvin, vin, startvin, i-1);
                root->right = helper(pre, startpre+i-startvin+1, endpre, vin, i+1, endvin);
                break;
            }
        }
        return root;
    }
};