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public class UniqueBinarySearchTreesII {
public List<TreeNode> generateTrees(int n) {
return helper(1, n);
}
private List<TreeNode> helper(int start, int end) {
List<TreeNode> result = new ArrayList<TreeNode>();
if (start > end) {
result.add(null);
return result;
}
for (int i = start; i <= end; i++) {
List<TreeNode> left = helper(start, i - 1);
List<TreeNode> right = helper(i + 1, end);
for (TreeNode l : left) {
for (TreeNode r : right) {
TreeNode root = new TreeNode(i);
root.left = l;
root.right = r;
result.add(root);
}
}
}
return result;
}
}
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