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

递归！

/**
 * 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 *> generate(int left, int right) {
        // The child of leaf is null.
        if(left > right) return vector<TreeNode *>(1, nullptr);
        
        // Get results, the rusults is the combination
        // of all possible left sub tree and right sub tree.
        vector<TreeNode *> results;
        for(int ii = left; ii <= right; ii ++) {
            // Get left sub trees and right sub trees.
            vector<TreeNode *> lefts = generate(left, ii - 1);
            vector<TreeNode *> rights = generate(ii + 1, right);
            // Combinate them, add the current as root.
            for(int jj = 0; jj < lefts.size(); jj ++) {
                for(int kk = 0; kk < rights.size(); kk ++) {
                    TreeNode *newroot = new TreeNode(ii);
                    newroot->left = lefts[jj];
                    newroot->right = rights[kk];
                    results.push_back(newroot);
                }
            }
        }
        return results;
    }
    vector<TreeNode *> generateTrees(int n) {
        return generate(1, n);
    }
};