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Solution.cs
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46 lines (44 loc) · 1.24 KB
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/**
* Definition for a binary tree node.
* public class TreeNode {
* public int val;
* public TreeNode left;
* public TreeNode right;
* public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
public class Solution
{
public IList<TreeNode> GenerateTrees(int n)
{
// n = 8
// O(2^8)
if (n <= 0) return [];
return Generate(1, n, []);
}
IList<TreeNode> Generate(int start, int end, Dictionary<(int, int), IList<TreeNode>> memo)
{
if (start > end) return [null];
var key = (start, end);
if (memo.TryGetValue(key, out IList<TreeNode> cached)) return cached;
IList<TreeNode> ret = [];
for (int i = start; i <= end; i++)
{
IList<TreeNode> left = Generate(start, i - 1, memo);
IList<TreeNode> right = Generate(i + 1, end, memo);
foreach (TreeNode leftNode in left)
{
foreach (TreeNode rightNode in right)
{
ret.Add(new(i, leftNode, rightNode));
}
}
}
memo[key] = ret;
return ret;
}
}