Flatten Binary Tree to Linked List

FlattenBinaryTreeToLinkedList.java

/*
Given a binary tree, flatten it to a linked list in-place.

For example,
Given

         1
        / \
       2   5
      / \   \
     3   4   6
The flattened tree should look like:
   1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6
*/

public void flatten(TreeNode root) {
    root = flatten(root, null);
}

TreeNode flatten(TreeNode cur, TreeNode next){
    if (cur == null) return next;
    next = flatten(cur.right, next);
    next = flatten(cur.left, next);
    cur.right = next;
    cur.left = null;
    return cur;
}

/*
 * Iterative version
 */
public void flatten(TreeNode root){
    if (root == null) return;
    TreeNode prev = null, cur = root;
    Stack<TreeNode> stack = new Stack<TreeNode>();
    stack.push(cur);
    while (!stack.isEmpty()){
        cur = stack.pop();
        if (cur.right != null)
            stack.push(cur.right);
        if (cur.left != null)
            stack.push(cur.left);
        if (prev != null){
            prev.right = cur;
            prev.left = null;
        }
        prev = cur;
    }
}