/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int ans=0;
    unordered_map<TreeNode*,int> m;
    int maxZigZag(TreeNode* root,int dir){

        if(root==nullptr){
            return 0;
        }

        if(dir==1){

            return 1+maxZigZag(root->left,0);

        }else{

            return 1+maxZigZag(root->right,1);

        }
    }


    void dfs(TreeNode* root){

        if(root==nullptr){
            return;
        }
        int a=0;
        if(m[root]==0){
            a=max(maxZigZag(root->left,0),maxZigZag(root->right,1));
            m[root]=a;
        }else{
            a=m[root];
        }


        ans=max(ans,a);

        dfs(root->right);
        dfs(root->left);
        return;
    }



    int longestZigZag(TreeNode* root) {

        dfs(root);

        return ans;
    }
};