LeetCode 95. Unique Binary Search Trees II(唯一二叉搜尋樹)
阿新 • • 發佈:2019-02-16
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
方法:遇到這種問題不能用深度優先,要用自頂向下,分而治之。
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { private List<TreeNode> generate(int from, int to) { List<TreeNode> roots = new ArrayList<>(); if (from == to) { roots.add(new TreeNode(from)); return roots; } for(int i=from; i<=to; i++) { if (i==from) { List<TreeNode> rights = generate(from+1, to); for(int j=0; j<rights.size(); j++) { TreeNode root = new TreeNode(from); root.right = rights.get(j); roots.add(root); } } else if (i==to) { List<TreeNode> lefts = generate(from, to-1); for(int j=0; j<lefts.size(); j++) { TreeNode root = new TreeNode(to); root.left = lefts.get(j); roots.add(root); } } else { List<TreeNode> lefts = generate(from, i-1); List<TreeNode> rights = generate(i+1, to); for(int j=0; j<lefts.size(); j++) { for(int k=0; k<rights.size(); k++) { TreeNode root = new TreeNode(i); root.left = lefts.get(j); root.right = rights.get(k); roots.add(root); } } } } return roots; } public List<TreeNode> generateTrees(int n) { return generate(1, n); } }