Unique Binary Search Trees II

描述​

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

For example, Given n = 3, your program should return all 5 unique BST's shown below.

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

代码​

// Unique Binary Search Trees II// 时间复杂度TODO，空间复杂度TODOpublic class Solution {    public List<TreeNode> generateTrees(int n) {        if (n == 0) return new ArrayList<>();        return generate(1, n);    }    private static List<TreeNode > generate(int start, int end) {        List<TreeNode> subTree = new ArrayList<>();        if (start > end) {            subTree.add(null);            return subTree;        }        for (int k = start; k <= end; k++) {            List<TreeNode> leftSubs = generate(start, k - 1);            List<TreeNode> rightSubs = generate(k + 1, end);            for (TreeNode i : leftSubs) {                for (TreeNode j : rightSubs) {                    TreeNode node = new TreeNode(k);                    node.left = i;                    node.right = j;                    subTree.add(node);                }            }        }        return subTree;    }}