Classic Unique Binary Search Trees II

Classic Unique Binary Search Trees II
 

Unique Binary Search Trees II

Original question:

Given n, generate all structurally unique BST's (binary search trees) that store values 1... n.

For example,
Given n = 3, your program shocould return all 5 unique BST's shown below.

   1         3     3      2      1           /     /      /            3     2     1      1   3      2    /     /                           2     1         2                 3

The most classic algorithm we have seen so far, the most popular solution with a score of over 14000:
 

/** * 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:    vector
 
   generateTrees(int n) {        return helper(1, n);    }    vector
  
    helper(int s, int e){        if(s>e){            return vector
   
    (1, NULL);        }                vector
    
      res;        for(int i=s; i<=e; i++){            vector
     
       left=helper(s, i-1);            vector
      
        right=helper(i+1, e); for(int j=0; j
       
        left=left[j]; p->right=right[k]; res.push_back(p); } } } return res; }};