mine （dp）

10.3

mine

思路：
简单dp

#include <cstdio>
#include <cstring>
#define LL long long 
#define N 5000010
const LL mod = 1e9 + 7;

int len;
char s[N];
LL f[N][5], ans = 0;
//5种状态 0：以0结尾   1：以*1结尾   2：以1结尾(1前无*)   3：以2结尾   4：以*结尾 

int main(){
    freopen("mine.in","r",stdin);
    freopen("mine.out","w",stdout);
    scanf("%s", s+1);
    len = strlen(s + 1);
    f[0][0] = 1;
    for(int i=1; i<=len; i++){
        if(s[i]=='?' || s[i]=='0') f[i][0] = (f[i-1][0] + f[i-1][1]) % mod;
        if(s[i]=='?' || s[i]=='1') f[i][1] = f[i-1][4], f[i][2] = (f[i-1][0] + f[i-1][1]) % mod;
        if(s[i]=='?' || s[i]=='2') f[i][3] = f[i-1][4];
        if(s[i]=='?' || s[i]=='*') f[i][4] = (f[i-1][4] + f[i-1][2] + f[i-1][3]) % mod;
        if((s[i]=='?' || s[i]=='*') && i == 1) f[i][4] = 1;
    }
    ans = (f[len][0] + f[len][1] + f[len][4]) % mod;
    printf("%I64d\n", ans);
    return 0;
}