Convert Binary Search Tree to Sorted Doubly-linked list

This is very interesting problem. And most of solutions are recursive approach which will cause stack overflow.

The below is my solution, which does in-place and iteratively.

The idea is to in-order traverse the tree and use head pointer referencing the head of the linked list and pre pointers referencing the previous node during traverse.

public static Node isBSTAndTreeToDLL(Node root){

if(root==null)

return null;

Stack<Node> s = new Stack<Node>();

Node head = null;

Node n = root;

Node pre = null;

while(!s.isEmpty() || n!=null){

if(n!=null){

s.push(n);

n = n.left;

}

else{

Node p = s.pop();

//this is the left most node

if(head == null)

head = p;

if(pre!=null){

pre.right = p;

p.left = pre;

}

pre=p;

n = p.right;

}

}

//handle the last node

pre.right = head;

head.left = pre;

return head;

}