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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int longestZigZag(TreeNode root) {
        if(root==null)
        {
            return 0;
        }
        return Math.max(1+helper(root.left,'l'),1+helper(root.right,'r'));
    }
    
    public int helper(TreeNode root,char c)
    {
        if(root==null)
        {
            return -1;
        }
        if(c=='l')
        {
            return Math.max(1+helper(root.right,'r'),0+helper(root.left,'l'));
        }
        else
        {
            return Math.max(1+helper(root.left,'l'),0+helper(root.right,'r'));
        }
    }
}