LeetCode-Unique Binary Search Trees II

生成二叉查找树；

哎，思考了大半天才写出来，

主要卡在生成结点上了，开始没考虑好怎么把不同的树在不同的地方复制前面的部分，

还是对递归的使用不够成熟，理解和掌握的还不够好！

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<TreeNode *> res;
        if (n <= 0) {
            res.push_back(NULL);
            return res;
        }
        generate(res, 1, n + 1);
        return res;
    }
    void generate(vector<TreeNode *> &res, int l, int r) {
        if (l == r) {
            res.push_back(NULL);
            return;
        }
        for (int i = l; i < r; ++i) {
            vector<TreeNode *> leftres;
            generate(leftres, l, i);
            vector<TreeNode *> rightres;
            generate(rightres, i + 1, r);
            for (int j = 0; j < leftres.size(); ++j) {
                for (int k = 0; k <rightres.size(); ++k) {
                    TreeNode *root = new TreeNode(i);
                    root->left = leftres[j];
                    root->right = rightres[k];
                    res.push_back(root);
                }
            }
        }
    }
};