Flatten Binary Tree to Linked List *

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    List<TreeNode> list = new ArrayList<TreeNode>();
    
    public void PreOrderTraverse(TreeNode root) {
        if(root!=null) {
            list.add(root);
            if(root.left!=null) PreOrderTraverse(root.left);
            if(root.right!=null) PreOrderTraverse(root.right);
        }
    }
    
    public void flatten(TreeNode root) {
        PreOrderTraverse(root);
        for(int i=0;i<list.size()-1;i++) {
            list.get(i).left = null;
            list.get(i).right =  list.get(i+1);
        }
    }
}