LeetCode – Refresh – Unique Binary Search Tree II

/**
 * 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 *> getTree(int start, int end) {
        vector<TreeNode *> result;
        if (start > end) {
            result.push_back(NULL);
        } else if (start == end) {
            result.push_back(new TreeNode(start));
        } else {
            for (int i = start; i <= end; i++) {
                vector<TreeNode *> t1 = getTree(start, i-1);
                vector<TreeNode *> t2 = getTree(i+1, end);
                for (int j = 0; j < t1.size(); j++) {
                    for (int k = 0; k < t2.size(); k++) {
                        TreeNode *root = new TreeNode(i);
                        root->left = t1[j];
                        root->right = t2[k];
                        result.push_back(root);
                    }
                }
            }
        }
        return result;
    }
    vector<TreeNode *> generateTrees(int n) {
        return getTree(1, n);
    }
};