Problem

Flatten a binary tree to a fake "linked list" in pre-order traversal.
Here we use the right pointer in TreeNode as the next pointer in ListNode.

Example

              1
               \
     1          2
    / \          \
   2   5    =>    3
  / \   \          \
 3   4   6          4
                     \
                      5
                       \
                        6
                        
                    

Solution

neat and beautiful

public class Solution {
    public void flatten(TreeNode root) {
        if (root == null) return;
        
        TreeNode left = root.left;
        TreeNode right = root.right;
        flatten(left);
        flatten(right);
        
        root.left = null;
        root.right = left;
        
        TreeNode cur = root;
        while (cur.right != null) cur = cur.right;
        
        cur.right = right;
    }
}

Use stack

    public void flatten(TreeNode root) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode p = root;
        while(p != null || !stack.empty()){
            if(p.right != null){
                stack.push(p.right);
            }
            if(p.left != null){
                p.right = p.left;
                p.left = null;
            }
            else if(!stack.empty()){
                TreeNode temp = stack.pop();
                p.right=temp;
            }
            p = p.right;
        }
    }