Codeforces 645E Intellectual Inquiry (dp不同子序列计数)

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
#include <map>
#include <vector>
using namespace std;

#define N 2000030
#define M 15000030
#define mod 1000000007


char s[N];
int n, k;
int pre[26];
int f[N];

int main() {
	scanf("%d%d", &n, &k);
	scanf("%s", s + 1);
	memset(pre, -1, sizeof pre);
	f[0] = 1;
	for(int i = 1; s[i]; ++i) {
		f[i] = 1;
		int c = s[i] - 'a';
		pre[c] = i;
		for(int j = 0; j < k; ++j) {
			if(pre[j] == -1) continue;
			f[i] += f[pre[j]-1];
			if(f[i] >= mod) f[i] -= mod;
		}
	}
	int m = strlen(s + 1);
	for(int i = m + 1; i <= m + n; ++i) {
		int mi = -1;
		f[i] = 1;
		for(int j = 0; j < k; ++j) {

			if(mi == -1 || pre[j] == -1 || pre[j] < pre[mi]) {
				mi = j;
			}
		}
		int c = mi;
		pre[c] = i;
		for(int j = 0; j < k; ++j) {
			if(pre[j] == -1) continue;
			f[i] += f[pre[j]-1];
			if(f[i] >= mod) f[i] -= mod;
		}
	}
	printf("%d\n", f[n+m]);
	return 0;
}