generate number from reverse linked list

Question

I have create a function int reverseNum(Node *head) to get number from reverse linked list. eg. for Linked List:- 1->2->3->4 the number will be 4321. But the program is returning 4320.

#include <bits/stdc++.h>
#define vi vector<int>


using namespace std;

struct Node {
    int data;
    Node *next;


    Node(int x,Node *n){
        data=x;
        next=n;
    }

    Node(int x){
        data=x;
        next=nullptr;
    }
};



//arr to LL
Node *arrToLL(vector<int> &arr){

    //chk if the arr is empty

    if (arr.empty()){
        return nullptr;
    }

    Node *head=new Node(arr[0]);

    Node *tmp=head;

    for (int i=1;i<arr.size();i++){
        Node *newNode=new Node(arr[i]);
        tmp->next=newNode;
        tmp=newNode;

    }


    return head;
}


//get reverse number from the LL

int reverseNum(Node *head){
    int ans=0;
    int exp=0;

    while (head!=nullptr){
        ans += (head->data * pow(10, exp)); 
        cout<<ans<<"-";
        exp++;

        head=head->next;
    }

    return ans;
}


int main(){
    vi arr1={1,2,3,4};
    vi arr2={3,5,6};

    Node *head1=arrToLL(arr1);
    Node *head2=arrToLL(arr2);

    cout<<reverseNum(head1);
    
    return 0;
}

Answer 1

The pow function returns a double type, as it uses floating point algorithmic. This can lead to imprecisions, like occurred for the user that asked Why does pow(n,2) return 24 when n=5, with my compiler and OS?.

You should not need to rely on floating point calculations for this. Instead of exp, use a variable that has the power you need, and which multiplies by 10 in each iteration:

int reverseNum(Node *head){
    int ans=0;
    int power=1; // <---

    while (head!=nullptr){
        ans += head->data * power;  // <---
        cout<<ans<<"-";
        power *= 10;  // <---
        head=head->next;
    }
    return ans;
}

Now there is no floating point processing in your code anymore and the output will be as expected.

NB: be aware that the linked list data structure could hold many more digits than int can represent. You will still get wrong results for linked lists that have like 30 digits.