I am getting "void value not ignored as it ought to be" what should i do?

Question

QUESTION LINK:

https://www.interviewbit.com/problems/minimize-the-absolute-difference/

It has an ide so you can solve it on the website itself.

QUESTION:

Given three sorted arrays A, B and C of not necessarily same sizes.

Calculate the minimum absolute difference between the maximum and minimum number from the triplet a, b, c such that a, b, c belongs arrays A, B, C respectively. i.e. minimize | max(a,b,c) - min(a,b,c) |.

Example :

Input:

A : [ 1, 4, 5, 8, 10 ]
B : [ 6, 9, 15 ]
C : [ 2, 3, 6, 6 ]

Output:

1

Explanation: We get the minimum difference for a=5, b=6, c=6 as | max(a,b,c) - min(a,b,c) | = |6-5| = 1.

MY SOLUTION:

int Solution::solve(vector<int> &A, vector<int> &B, vector<int> &C) {

    int a,b,c,d,e,f,g,i,j;

    a=A.pop_back();
    b=B.pop_back();
    c=C.pop_back();

    while(A.size()>0 && B.size()>0 && C.size()>0){


        if(a>=b && b>=c){
            d = abs(a-c);
            if(d<f){
                f=d;
            }
            a=A.pop_back();
        }

        if(a>=b && c>=b){
            d = abs(a-b);
            if(d<f){
                f=d;
            }
            a=A.pop_back();
        }

        if(b>=a && a>=c){
            d = abs(b-c);
            if(d<f){
                f=d;
            }
            b=B.pop_back();
        }

        if(b>=a && c>=a){
            d = abs(b-a);
            if(d<f){
                f=d;
            }
            b=B.pop_back();
        }

        if(c>=a && a>=b){
            d = abs(c-b);
            if(d<f){
                f=d;
            }
            c=C.pop_back();
        }

        if(c>=b && b>=a){
            d = abs(c-a);
            if(d<f){
                f=d;
            }
            c=C.pop_back();
        }


    }

    cout<<f;

}

Answer 1

The pop_back() function doesn't return the element popped. You have to do this:

a = A.back();
A.pop_back();

... and so on.

See Why doesn't std::queue::pop return value.? for explanation of the API design.