[LeetCode] Problem 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.

Example

Input: 3

Output:

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[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]

Explanation:
The above output corresponds to the 5 unique BST’s shown below:

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1         3     3      2      1
 \       /     /      / \      \
  3     2     1      1   3      2
 /     /       \                 \
2     1         2                 3

Code

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public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}
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public List<TreeNode> generateTrees(int n) {   
    if (n == 0)
        return new ArrayList<>();
    
    return generate(1, n);
}

private List<TreeNode> generate(int start, int end) {
    List<TreeNode> result = new ArrayList<>();

    if (start > end) {
        result.add(null);
        return result;
    }

    for (int i = start; i <= end; i++) {
        List<TreeNode> leftList = generate(start, i - 1);
        List<TreeNode> rightList = generate(i + 1, end);

        for (TreeNode left : leftList) {
            for (TreeNode right : rightList) {
                TreeNode root = new TreeNode(i);
                root.left = left;
                root.right = right;
                result.add(root);
            }
        }
    }

    return result;
}