为什么当我从语句检查中删除"+ mod"时，以下程序给出错误的答案。问题链接：https://www.codechef.com/problems/FFC219B

语句检查是我不明白为什么当我写"sum = (solution[R]-solution[L-1](%mod;"时它在提交时显示错误的答案。在这里我没有在括号内添加 mod。我不明白答案如何通过添加取相同模组的值来改变。代码厨师中的问题代码：https://www.codechef.com/problems/FFC219B

#include<iostream>
#define ll long long
#define mod 1000000007   //the modulus we need to take for the final answer
#define endl "n"
using namespace std;
long long solution[100007] = {0}; //Initialising all the values with zero
int main(){
ios_base :: sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
solution[0] = 0;
ll a1=1,a2=2,a3=3,a4=4;    //The variable initialising as per the problem
for(int i = 1;i <= 100007;i++){
ll k=(a1 * a2) % mod * a3 % mod * a4 % mod;
solution[i] = (solution[i-1]+k)%mod;  //Adding the previous values as we are to find the sum in range
a1++;
a2++;
a3++;
a4++;
}
int t; //Taking input for number of test cases
cin>>t;
while(t-->0)
{
int L,R;
cin>>L>>R; //Taking the range input
long long sum = 0;
sum = (solution[R]-solution[L-1] + mod)%mod; //statement check & final answer
cout<<sum<<endl;
}
return 0;
}