[leetcode-95-Unique Binary Search Trees II]
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Given an integer 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:
This problem is a variant of the problem of Unique Binary Search Trees.
I provided a solution along with explanation for the above problem, in the question "DP solution in 6 lines with explanation"
It is intuitive to solve this problem by following the same algorithm. Here is the code in a divide-and-conquer style.
public List<TreeNode> generateTrees(int n) { return generateSubtrees(1, n); } private List<TreeNode> generateSubtrees(int s, int e) { List<TreeNode> res = new LinkedList<TreeNode>(); if (s > e) { res.add(null); // empty tree return res; } for (int i = s; i <= e; ++i) { List<TreeNode> leftSubtrees = generateSubtrees(s, i - 1); List<TreeNode> rightSubtrees = generateSubtrees(i + 1, e); for (TreeNode left : leftSubtrees) { for (TreeNode right : rightSubtrees) { TreeNode root = new TreeNode(i); root.left = left; root.right = right; res.add(root); } } } return res; }
参考:
https://discuss.leetcode.com/topic/8410/divide-and-conquer-f-i-g-i-1-g-n-i
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Leetcode 95. Unique Binary Search Trees II
LeetCode 95. Unique Binary Search Trees II
[leetcode-95-Unique Binary Search Trees II]
leetcode95 Unique Binary Search Trees II