leetcode 95. Different Binary Search Trees II

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/**
 * 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) {
        vector<TreeNode*> res;
        if(n < 1) return res;
        return f(1, n);
    }
    vector<TreeNode*> f(int l, int r) {
        vector<TreeNode*> res;
        if(l > r) res.push_back(NULL);
        else {
            for(int i = l; i <= r; i++) {
                vector<TreeNode*> left = f(l, i - 1);
                vector<TreeNode*> right = f(i + 1, r);
                for(auto a : left) {
                    for(auto b : right) {
                        TreeNode* root = new TreeNode(i);
                        root->left = a;
                        root->right = b;
                        res.push_back(root);
                    }
                }
            }
        }
        return res;
    }
};