C++ Solution using constant space;

#1 
void Solution::connect(TreeLinkNode* A) {
    if(A==NULL) return;

    TreeLinkNode *prevst = A;
    TreeLinkNode *node,*prev;

    while(prevst!=NULL){
        node = prevst;
        prevst = NULL;
    
        while(node!=NULL){
        
            if(node->left!=NULL){
                if(prevst==NULL){
                    prev = node->left;
                    prevst = prev;
                }
                else{ 
                    prev->next = node->left;
                    prev = prev->next;
                }
            }
        
            if(node->right!=NULL){
                if(prevst==NULL){
                    prev = node->right;
                    prevst = prev;
                }
                else{ 
                    prev->next = node->right;
                    prev = prev->next;
                }
            }
        
            node=node->next;
        }       
    }
}