#include<bits/stdc++.h>
using namespace std;
#define int long long
const int N = 1e6 + 10;
const int MOD = 1e9 + 7 , mod = 999998639;
int k[N] , n , p;
int pow_mod(int x , int n , int mod)
{
int res = 1;
while(n)
{
if(n & 1) res = res * x % mod;
x = x * x % mod;
n >>= 1;
}
return res;
}
signed main()
{
ios::sync_with_stdio(false);
int t;
cin >> t;
while(t --)
{
cin >> n >> p;
for(int i = 1 ; i <= n ; i ++) cin >> k[i];
sort(k + 1 , k + 1 + n , greater<int>());
int ans1 = 0 , ans2 = 0;
for(int i = 1 ; i <= n ; i ++)
{
if(!ans1 && !ans2)
ans1 += pow_mod(p , k[i] , MOD),
ans2 += pow_mod(p , k[i] , mod);
else
ans1 = (ans1 - pow_mod(p , k[i] , MOD) + MOD) % MOD,
ans2 = (ans2 - pow_mod(p , k[i] , mod) + mod) % mod;
}
cout << ans1 << '\n';
}
return 0;
}
Can somebody explain why this solution works for last (DIV2E).It really feels easy to code but what is logic behind another mod ?? https://codeforces.net/contest/1361/problem/B