2327. Number of people who know secrets (recursive)

2327. Number of people who know the secret ( Recurrence )

A one-dimensional dp Can qwq. Make $f_i$ It means the first one $i$ Tianxin knows the number of Secrets .

The first $i$ Days can be controlled by $(i-forget,i-delay]$ Recurrence .

Obviously, prefixes and , Achieve $O(n)$

The final answer is ：$\sum _{i=n-forget+1}^n{f}_i$

class Solution {
    
    const int MOD = 1000000007;

public:
    int peopleAwareOfSecret(int n, int delay, int forget) {
    
        vector<long long> f(n + 1), g(n + 1);
        f[1] = g[1] = 1;
        for (int i = 2; i <= n; i++) {
    
            int L = max(0, i - forget);
            int R = max(0, i - delay);
            f[i] = (g[R] - g[L] + MOD) % MOD;
            g[i] = (g[i - 1] + f[i]) % MOD;
        }
        return (g[n] - g[max(0, n - forget)] + MOD) % MOD;
    }
};