2327.Number-of-People-Aware-of-a-Secret_v2.cpp

using LL = long long;
LL M = 1e9+7;
class Solution {
public:
    int peopleAwareOfSecret(int n, int delay, int forget) 
    {        
        vector<LL>dp(n+1); // dp[i]: # of new persons who know the news in the i-th day
        vector<LL>diff(n+1);
        
        dp[1] = 1;
        diff[1] += 1;
        diff[2] += -1;
        
        LL ret = 0;
        for (int i=1; i<=n; i++)
        {
            dp[i] = (dp[i-1] + diff[i] + M) % M;

            if (i+delay <= n)
                diff[i+delay] += dp[i];
            if (i+forget <= n)
                diff[i+forget] -= dp[i];
        }
        
        for (int i=1; i<=n; i++)
            if (i+forget > n)
                ret = (ret + dp[i]) % M;
        return ret;
    }
};