Plus One Linked List in C++

Suppose we have a non-negative integer represented as non-empty a singly linked list of digits, now we have to plus one to the integer. We may assume the integer do not contain any leading zero, except the number 0 itself. In the linked list the most significant digit is at the head of the list.

So, if the input is like [1,2,3], then the output will be [1,2,4]

To solve this, we will follow these steps −

• if head is null, then −

  • return head

• curr = head

• req = NULL

• while curr is non-zero, do −

  • if val of curr is not equal to 9, then −

    • req := curr

  • curr := next of curr

• if not req is non-zero, then −

  • dummy = new node with value 1

  • next of dummy := head

  • while head is non-zero, do −

    • val of head := 0

    • head := next of head

  • return dummy

• Otherwise

  • increase val of req by 1

  • req := next of req

  • while req is non-zero, do −

    • val of req := 0

    • req := next of req

  • return head

Example 

Let us see the following implementation to get a better understanding.

 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* plusOne(ListNode* head) {
      if (!head)
         return head;
      ListNode* curr = head;
      ListNode* req = NULL;
      while (curr) {
         if (curr->val != 9) {
            req = curr;
         }
         curr = curr->next;
      }
      if (!req) {
         ListNode* dummy = new ListNode(1);
         dummy->next = head;
         while (head) {
            head->val = 0;
            head = head->next;
         }
         return dummy;
      }
      else {
         req->val++;
         req = req->next;
         while (req) {
            req->val = 0;
            req = req->next;
         }
         return head;
      }
   }
};
main(){
   Solution ob;
   vector<int< v = {1,4,5};
   ListNode *head = make_list(v);
   print_list(ob.plusOne(head));
}

Input

{1,4,5}

Output

[1, 4, 6, ]