Flatten Binary Tree to Linked List

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

click to show hints.

Hints:

If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

Analysis: The same as pre-order traversal, i.e. use a stack. Let the right child pointers of each node point to the next node of the pre-order traversal of this tree. 

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void flatten(TreeNode root) {
        if(root==null) return;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while(!stack.isEmpty()) {
            TreeNode cur = stack.pop();
            if(cur.right!=null) stack.push(cur.right);
            if(cur.left!=null) stack.push(cur.left);
            if(!stack.isEmpty()) cur.right=stack.peek();    // right pointer always points to the next node in pre-order traversal
            cur.left = null;
        }
        
        return;
    }
}