/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* constructFromPrePost(vector<int>& pre, vector<int>& post) {
        return helper(pre, 0, pre.size()-1, post, 0, post.size()-1);
    }
    TreeNode* helper(vector<int>& pre, int s1, int e1, vector<int>& post, int s2, int e2){
        if(s1>e1 || s2>e2) return NULL;
        TreeNode* cur = new TreeNode(pre[s1]);
        int cnt=0;
        for(int i=s2;i<e2;i++){
            cnt++;
            if(post[i]==pre[s1+1] && post[e2-1]==pre[s1-s2+i+2]) break;
        }
        cur->left = helper(pre, s1+1, s1+cnt, post, s2, s2+cnt-1);
        cur->right = helper(pre, s1+1+cnt, e1, post, s2+cnt, e2-1);
        return cur;
    }
};