/*
// Definition for a Node.
class Node {
public:
    int val;
    Node* left;
    Node* right;
    Node* next;

    Node() : val(0), left(NULL), right(NULL), next(NULL) {}

    Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}

    Node(int _val, Node* _left, Node* _right, Node* _next)
        : val(_val), left(_left), right(_right), next(_next) {}
};
*/
class Solution {
public:
    int flag = 1;
    Node* connect(Node* root) {
        if(!root) return NULL;
        if(flag){
            root->next = NULL;
            flag = 0;
        }
        if(!(root->left)){
            return root;
        }
        else root->left->next = root->right;
        if(root->next){
            root->right->next = root->next->left;
        }else{
            root->right->next = nullptr;
        }
        connect(root->left);
        connect(root->right);
        return root;
    }
};