//----------------------------------------------------------------------------------------------------
// O(1) space method: change structure of node with next link in both directions
public static List<int> Traverse(PowerTreeNode root) {
if (root == null) return new List<int>();
LinkNextRight(root);
LinkNextLeft(root);
List<int> res = new List<int>();
// true for dirction from right to left, false for direction from left to right
bool leftDir = true;
PowerTreeNode curr = root;
PowerTreeNode head = root.left != null ? root.left : root.right;
bool gotHead = false; // flag for finding head node in next level
while (head != null) {
while (curr != null) {
res.Add(curr.val);
if (leftDir)
{
PowerTreeNode temp = curr.left != null ? curr.left : curr.right;
if (temp != null) {
head = temp;
gotHead = true;
}
curr = curr.leftNext;
}
else
{
PowerTreeNode temp = curr.right != null ? curr.right : curr.left;
if (temp != null) {
head = temp;
gotHead = true;
}
curr = curr.rightNext;
}
if (!gotHead)
head = null;
}
leftDir = !leftDir;
curr = head;
gotHead = false;
}
return res;
}
// link nodes in same level in <- direction
private static void LinkNextLeft(PowerTreeNode root)
{
if (root == null)
return;
PowerTreeNode curr = root;
// head node in next level
PowerTreeNode head = root.right != null ? root.right : root.left;
PowerTreeNode prev = null; // previous node in next level
// when next level exists
while (head != null)
{
// walk through current level and link nodes in next level
while (curr != null)
{
if (prev == null)
{
head = curr.right != null ? curr.right : curr.left;
prev = head;
}
if (curr.right != null && curr.right != prev)
{
prev.leftNext = curr.right;
prev = curr.right;
}
if (curr.left != null && curr.left != prev)
{
prev.leftNext = curr.left;
prev = curr.left;
}
curr = curr.leftNext;
}
// move to next level
curr = head;
prev = null;
}
}
