leetcode Unique Binary Search Trees II

UniqueBinarySearchTreesII.cpp

/*
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 binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
 class Solution {
 public:

    void generate(vector<TreeNode *> &ret, int left, int right) {
        if(left > right) {
            ret.push_back(NULL);
            return;
        }
        
        for(int i = left; i <= right; i++) {
            vector<TreeNode *> lefts;
            generate(lefts, left, i-1);
            
            vector<TreeNode *> rights;
            generate(rights, i+1, right);

            for(int u = 0; u < lefts.size(); u++) {
                for(int v = 0; v < rights.size(); v++) {
                    TreeNode *root = new TreeNode(i);
                    root->left = lefts[u];
                    root->right = rights[v];
                    ret.push_back(root);
                }
            }
        }
    }

    vector<TreeNode *> generateTrees(int n) {
        vector<TreeNode *> ret;
        generate(ret, 1, n);
        return ret;  
    }
};