LeetCode-95. Unique Binary Search Trees II

Given an integer n, generate all structurally unique BST's (binary search trees) that store values 1 ... n.

Example:

Input: 3
Output:
[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]
Explanation:
The above output corresponds to the 5 unique BST's shown below:

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3

题解：类似894题，分治法，把左右所有可能的子树求出来，然后组装。

class Solution {
public:
    vector<TreeNode*> generateTrees(int left, int right) {
        if (left > right) {
            return {NULL};
        }
        if (left == right) {
            return {new TreeNode(left)};
        }
        vector<TreeNode*> res;
        for (int k = left; k <= right; k++) {
            vector<TreeNode*> lTrees = generateTrees(left, k - 1);
            vector<TreeNode*> rTrees = generateTrees(k + 1, right);
            for (int i = 0; i < lTrees.size(); i++) {
                for (int j = 0; j < rTrees.size(); j++) {
                    TreeNode* idx = new TreeNode(k);
                    idx->left = lTrees[i];
                    idx->right = rTrees[j];
                    res.push_back(idx);
                }
            }
        }
        return res;
    }
    vector<TreeNode*> generateTrees(int n) {
        if (n == 0) {
            return {};
        }
        return generateTrees(1, n);
    }
};