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
解释:n的答案依赖于0~n-1的答案
public List<TreeNode> generateTrees(int n) {
//a list to store all the answers, use list instead of map is because we need to use them by indexing the n
List<List<TreeNode>> allResult = new ArrayList<>();
//initialize the n = 0 situation
List<TreeNode> list = new ArrayList<