LeetCode——Unique Binary Search Trees II

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

原题链接：https://oj.leetcode.com/problems/unique-binary-search-trees-ii/

思路： 与之前的求二叉搜索树的数量的题一样的思路，只是这里要实际构建树。因为二叉搜索树是这样的树，左节点小于根，根小于右节点。所以任选一个节点 i 作为根，则(1,i) 个数可作为左子树的节点，(i+1,n)个数可作为右子树的节点。所以，只要遍历所有的i，就可以得出所有的树。

public List<TreeNode> generateTrees(int n) {return generateTrees(1,n);}public List<TreeNode> generateTrees(int start,int end) {List<TreeNode> list = new ArrayList<TreeNode>();if(start>end){list.add(null);return list;}for(int i=start;i<=end;i++){List<TreeNode> lefts = generateTrees(start,i-1);List<TreeNode> rights = generateTrees(i+1,end);for(TreeNode left : lefts){for(TreeNode right : rights){TreeNode node = new TreeNode(i);node.left = left;node.right = right;list.add(node);}}}return list;}// Definition for binary treepublic class TreeNode {int val;TreeNode left;TreeNode right;TreeNode(int x) {val = x;}}