leetcode: 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

分别选一个数做根节点，然后把剩下比它小的放左边，比它大的放右边，然后枚举左右节点不同的放置情况。

/** * Definition for binary tree * 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 createTrees( 1, n);    }    vector<TreeNode *> createTrees( int left, int right){        vector< TreeNode *> res;        if( left > right){            res.push_back(NULL);            return res;        }        for( int i = left; i <= right; ++i){            vector< TreeNode *> l = createTrees( left, i-1);            vector< TreeNode *> r = createTrees( i+1, right);            for( int j = 0; j < l.size(); ++j){                for( int k = 0; k < r.size(); ++k){                    TreeNode * root = new TreeNode(i);                    root->left = l[j];                    root->right = r[k];                    res.push_back(root);                }            }        }        return res;    }};