Flatten Binary Tree to Linked List.java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    static TreeNode lastVisitedNode = null;
    public void flatten(TreeNode root) {
        if (root == null) {
            return;
        }
        
        TreeNode right = root.right;
        if (lastVisitedNode != null) {
            lastVisitedNode.left = null;
            lastVisitedNode.right = root;
        }
        lastVisitedNode = root;
        
        flatten(root.left);
        flatten(right);
    }
}