leetcode-----95. 不同的二叉搜索树 II

• 链接：https://leetcode-cn.com/problems/unique-binary-search-trees-ii/

代码

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<TreeNode*> generateTrees(int n) {
        if (n == 0) return {};
        return dfs(1, n);   
    }

    vector<TreeNode*> dfs(int l, int r) {
        if (l > r) return {NULL};
        vector<TreeNode*> ans;

        for (int i = l; i <= r; ++i) {
            auto left = dfs(l, i - 1), right = dfs(i + 1, r);
            for (auto l: left) {
                for (auto r: right) {
                    auto root = new TreeNode(i);
                    root->left = l, root->right = r;
                    ans.push_back(root);
                }
            }
        }
        return ans;
    }
};