95. Unique Binary Search Trees II 独一无二的二叉搜索树之二

[GH1995]

95. Unique Binary Search Trees II

Difficulty: Medium

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

Solution

Language: C++

class Solution {
public:
    vector<TreeNode *> generateTrees(int n) {
        if (n == 0) return {};
​
        return dfs(1, n);
    }
​
    vector<TreeNode *> dfs(int start, int end) {
        if (start > end) return {nullptr};
        vector<TreeNode *> result;
​
        for (int i = start; i <= end; ++i) {
            auto left = dfs(start, i - 1);
            auto right = dfs(i + 1, end);
​
            for (auto a : left)
                for (auto b:right) {
                    auto node = new TreeNode(i);
                    node->left = a;
                    node->right = b;
                    result.push_back(node);
                }
        }
​
        return result;
    }
};