95.Unique-Binary-Search-Trees-II

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https://leetcode.com/problems/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:
[
[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:

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

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Approach #1 Recursion

``````/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<TreeNode> generateTrees(int n) {
if (n = 0) {
}
return generate_trees(1, n);
}

public LinkedList<TreeNode> generate_trees(int start, int end) {
if (start > end) {
return all_trees;
}

for (int i = start; i <= end; i++) {
LinkedList<TreeNode> left_trees = generate_trees(start, i - 1);
LinkedList<TreeNode> right_trees = generate_trees(i + 1, end);

for (TreeNode l: left_trees) {
for (TreeNode r: right_trees) {
TreeNode current_tree = new TreeNode(i);
current_tree.left = l;
current_tree.right = r;