【Leetcode】Unique Binary Search Trees II

题目链接：https://leetcode.com/problems/unique-binary-search-trees-ii/
题目：

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

思路：

当根结点分别为1~n时，递归遍历，根结点大于左子树小于右子树。

算法：

public List<TreeNode> generateTrees(int n) {          if (n == 0)              return null;          return dsp(1, n);      }        public List<TreeNode> dsp(int start, int end) {          List<TreeNode> res = new ArrayList<TreeNode>();          if (start > end) {              res.add(null);              return res;          }            for (int i = start; i <= end; i++) {              List<TreeNode> left = dsp(start, i - 1);              List<TreeNode> right = dsp(i + 1, end);              for (TreeNode l : left) {                  for (TreeNode r : right) {                      TreeNode root = new TreeNode(i);                      root.left = l;                      root.right = r;                      res.add(root);                  }              }          }          return res;      }