Sum of two arrays

#1


run time error

#2 
import java.util.*;
public class Main {
    public static void main (String args[]) {
        Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int arr1[] = new int[n];
		for (int i = 0; i < arr1.length; i++) {
			arr1[i] = sc.nextInt();

		}

		int m = sc.nextInt();
		int arr2[] = new int[m];

		for (int j = 0; j < arr2.length; j++) {
			arr2[j] = sc.nextInt();

		}
        
		if(n>m) {
			sum(arr1,arr2,n,m);
		}else {
			sum(arr2,arr1,m,n);
		}
	}
	
	
	public static void sum(int a[], int []b, int n , int m ) {
		int carry =0,s=0;
		int i=n-1;
		int j=m-1;
		int k=n-1;
		int sum[] = new int[k+1];//you should make an array of k+1 size
		while(j>=0) {
			s=a[i]+b[j]+carry;
			sum[k]=s%10;
			carry = s/10;
			i--;
			j--;
			k--;
		}
		
		while(i>=0) {
			s=a[i]+carry;
			sum[k]=s%10;
			carry=s/10;
			i--;
			k--;
		}
		
		
		if(carry==1) {
			System.out.print(1+", ");//if there is a carry then dont add 10 just print 1 do a dry run for 99 and 999
		}
		for(int r =0;r<n;r++)
		{
			System.out.print(sum[r]+", ");
		}
		System.out.println("END");
    }
}
#3

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#4

I didn’t get it, why did we do k=n-1? and why not k=m-1 because currently we are not sure right which array is bigger?