经典中的经典Unique Binary Search Trees II

Unique Binary Search Trees II

原题：

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

 迄今为止看到的最为经典的算法，分数超过14000的好评

最经典的解法：

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

来自leetcode：https://leetcode.com/discuss/26330/30-ms-c-solution