經典中的經典Unique Binary Search Trees II

Unique Binary Search Trees II

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

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

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

/**
 * 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; jleft=left[j];
                    p->right=right[k];
                    res.push_back(p);
                }
            }
        }
        return res;
    }
};