Leetcode 95. Unique Binary Search Trees II

原创于 2016-04-05 19:08:54 发布 · 475 阅读

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给定整数n，生成所有1到n之间值的结构独特二叉搜索树。例如，当n为3时，将返回5种不同的BST结构。

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Question

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

Code

public List<TreeNode> get(int left, int right) {
        List<TreeNode> res = new ArrayList<>();
        if (left <= right) {
            for (int i = left; i <= right; i++) {
                List<TreeNode> l = get(left, i - 1);
                List<TreeNode> r = get(i + 1, right);
                for (TreeNode ll : l) {
                    for (TreeNode lr : r) {
                        TreeNode s = new TreeNode(i);
                        s.left = ll;
                        s.right = lr;
                        res.add(s);
                    }
                }
            }

        } else {
            res.add(null);
        }
        return res;
    }

    public List<TreeNode> generateTrees(int n) {
        if (n == 0) {
            return new ArrayList<>();
        }
        return get(1, n);
    }