Remove Zero Sum Consecutive Nodes from Linked List in C++

Suppose we have given the head of a linked list; we have to repeatedly delete consecutive sequences of nodes that sum to 0 until there are no such sequences. So after doing so, we have to return the head of the final linked list. So if the list is like [1,2,-3,3,1], then the result will be [3,1].

To solve this, we will follow these steps −

• Create a node called dummy, and store 0 into it, set next of dummy := head

• create one map m, store dummy for the key 0 into m, set sum = 0

• while head is not null −

  • sum := sum + value of head, set m[sum] := head, and head := next of head

• head := dummy

• sum := 0

• while head is not null

  • sum := sum + value of head

  • temp := m[sum]

  • if temp is not head, then next of head := next of temp

  • head := next of head

• return next of dummy

Let us see the following implementation to get better understanding −

Example

 Live Demo

#include <bits/stdc++.h>
using namespace std;
class ListNode{
   public:
   int val;
   ListNode *next;
   ListNode(int data){
      val = data;
      next = NULL;
   }
};
ListNode *make_list(vector<int> v){
   ListNode *head = new ListNode(v[0]);
   for(int i = 1; i<v.size(); i++){
      ListNode *ptr = head;
      while(ptr->next != NULL){
         ptr = ptr->next;
      }
      ptr->next = new ListNode(v[i]);
   }
   return head;
}
void print_list(ListNode *head){
   ListNode *ptr = head;
   cout << "[";
   while(ptr){
      cout << ptr->val << ", ";
      ptr = ptr->next;
   }
   cout << "]" << endl;
}
class Solution {
   public:
   ListNode* removeZeroSumSublists(ListNode* head) {
      ListNode* dummy = new ListNode(0);
      dummy->next = head;
      unordered_map <int, ListNode*> m;
      m[0] = dummy;
      int sum = 0;
      while(head){
         sum += head->val;
         m[sum] = head;
         head = head->next;
      }
      head = dummy;
      sum = 0;
      while(head){
         sum += head->val;
         ListNode* temp = m[sum];
         if(temp != head){
            head->next = temp->next;
         }
         head = head->next;
      }
      return dummy->next;
   }
};
main(){
   vector<int> v1 = {1,2,-3,3,1};
   ListNode *head = make_list(v1);
   Solution ob;
   print_list(ob.removeZeroSumSublists(head));
}

Input

[1,2,-3,3,1]

Output

[3,1]