Add 1 to a number represented as linked list?

Here we will see how to add 1 with a number stored into a linked list. In the linked list, each digit of the numbers is stored. If the number is 512, then it will be stored like below −

512 = (5)-->(1)-->(2)-->NULL

We are providing the list into the increment function. That will return another list after adding 1 with it. Here we are using the C++ STL linked list. Let us see the algorithm to bet better idea.

Algorithm

incrementList(l1)

Begin
   carry := 1
   res := an empty list
   for each node n from l1, scan from last to first, do
      item := (l1.item + carry) mod 10
      insert item at the beginning of res
      carry := (l1.item + carry) / 10
   done
   if carry is not 0, then
      add carry at the beginning of res
   end if
   return res
End

Example

#include<iostream>
#include<list>
using namespace std;
list<int> incListNum(list<int> l1){
   list<int>::reverse_iterator it1 = l1.rbegin();
   list<int> result;
   int carry = 1; //this 1 will be added
   while(it1 != l1.rend()){
      result.push_front((*it1 + carry) % 10);
      carry = (*it1 + carry) / 10;
      it1++;
   }
   if(carry != 0){
      result.push_front(carry);
   }
   return result;
}
list<int> numToList(int n){
   list<int> numList;
   while(n != 0){
      numList.push_front(n % 10);
      n /= 10;
   }
   return numList;
}
void displayListNum(list<int> numList){
   for(list<int>::iterator it = numList.begin(); it != numList.end();
   it++){
      cout<<*it;
   }
   cout << endl;
}
int main() {
   int n1 = 9999;
   list<int> n1_list = numToList(n1);
   list<int> res = incListNum(n1_list);
   cout << "The number: "; displayListNum(n1_list);
   cout << "Result: "; displayListNum(res);
}

Output

The number: 9999
Result: 10000

Arnab Chakraborty

Published on 31-Jul-2019 12:20:04

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