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/**

* Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

*/

import java.util.ArrayList;

import java.util.List;

/*

The following classes are defined in seperate files.

*/

/**

* Definition for singly-linked list.

* public class ListNode {

* int val;

* ListNode next;

* ListNode(int x) { val = x; }

* }

*/

/**

* Definition for a binary tree node.

* public class TreeNode {

* int val;

* TreeNode left;

* TreeNode right;

* TreeNode(int x) { val = x; }

* }

*/

public class ConvertSortedListToBST109 {

public TreeNode sortedListToBST(ListNode head) {

List<TreeNode> nodes = new ArrayList<TreeNode>();

storeBSTNodes(head, nodes);

int n = nodes.size();

return buildBST(nodes, 0, n-1);

}

private void storeBSTNodes(ListNode head, List<TreeNode> nodes) {

while (head != null) {

nodes.add(new TreeNode(head.val));

head = head.next;

}

}

private TreeNode buildBST(List<TreeNode> nodes, int start, int end) {

if (start > end)

return null;

int mid = (start + end) / 2;

TreeNode node = nodes.get(mid);

node.left = buildBST(nodes, start, mid - 1);

node.right = buildBST(nodes, mid + 1, end);

return node;

}

/** -------------------------------------------------------------------

* Top Solution: O(n logn)

* https://discuss.leetcode.com/topic/35997/share-my-java-solution-1ms-very-short-and-concise

* --------------------------------------------------------------------

*/

public TreeNode sortedListToBST2(ListNode head) {

if (head == null) return null;

return toBST(head, null);

}

public TreeNode toBST(ListNode head, ListNode tail){

ListNode slow = head;

ListNode fast = head;

if(head == tail) return null;

while(fast != tail && fast.next != tail){

fast = fast.next.next;

slow = slow.next;

}

TreeNode thead = new TreeNode(slow.val);

thead.left = toBST(head, slow);

thead.right = toBST(slow.next, tail);

return thead;

}

/** -------------------------------------------------------------------

* Top Solution: O(n)

* https://discuss.leetcode.com/topic/35997/share-my-java-solution-1ms-very-short-and-concise

* --------------------------------------------------------------------

*/

private ListNode node;

public TreeNode sortedListToBST3(ListNode head) {

if(head == null){

return null;

}

int size = 0;

ListNode runner = head;

node = head;

while(runner != null){

runner = runner.next;

size ++;

}

return inorderHelper(0, size - 1);

}

public TreeNode inorderHelper(int start, int end){

if(start > end){

return null;

}

int mid = start + (end - start) / 2;

TreeNode left = inorderHelper(start, mid - 1);

TreeNode treenode = new TreeNode(node.val);

treenode.left = left;

node = node.next;

TreeNode right = inorderHelper(mid + 1, end);

treenode.right = right;

return treenode;

}

public TreeNode sortedListToBST4(ListNode head) {

if (head == null) return null;

if (head.next == null) return new TreeNode(head.val);

ListNode fast = head;

ListNode slow = head;

ListNode beforeSlow = null;

while (slow != null && fast != null && fast.next != null) {

fast = fast.next.next;

beforeSlow = slow;

slow = slow.next;

}

beforeSlow.next = null;

TreeNode res = new TreeNode(slow.val);

res.left = sortedListToBST(head);

res.right = sortedListToBST(slow.next);

return res;

}

public TreeNode sortedListToBST5(ListNode head) {

if (head == null) return null;

ListNode fast = head;

ListNode slow = head;

ListNode pre = null;

while (fast != null && fast.next != null) {

fast = fast.next.next;

pre = slow;

slow = slow.next;

}

if (pre == null) {

return new TreeNode(head.val);

}

pre.next = null;

TreeNode root = new TreeNode(slow.val);

root.left = sortedListToBST(head);

root.right = sortedListToBST(slow.next);

return root;

}

}