Flatten Tree In-Order

FlattenTreeInOrder.java

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