Codeforces 1045H Self-exploration 思维

#include<bits/stdc++.h>
using namespace std;

const int N = (int)1e5 + 7;
const int mod = (int)1e9 + 7;

int inv[N], F[N], Finv[N], c[2][2], cc[2][2];
int n, len;
char s[N], t[N];


inline int C(int n, int m) {
    if(n == -1 && m == -1) return 1;
    if(n < 0 || n < m) return 0;
    return 1LL * F[n] * Finv[m] % mod * Finv[n - m] % mod;
}

inline int getAns0(int c[2][2], int n) {
    if(c[0][1] < c[1][0] || c[0][1] > c[1][0] + 1) return 0;
    for(int i = 0; i < 2; i++) {
        for(int j = 0; j < 2; j++) {
            if(c[i][j] < 0) return 0;
        }
    }
    int seg0, seg1;
    if(c[0][1] == c[1][0]) seg0 = c[0][1] + 1, seg1 = c[0][1];
    else seg0 = c[0][1], seg1 = c[0][1];
    int cnt0 = c[0][0] + seg0;
    int cnt1 = c[1][1] + seg1;
    if(cnt0 + cnt1 != n) return 0;
    return 1LL * C(cnt0 - 1, seg0 - 1) * C(cnt1 - 1, seg1 - 1) % mod;
}

inline int getAns1(int c[2][2], int n) {
    if(c[1][0] < c[0][1] || c[1][0] > c[0][1] + 1) return 0;
    for(int i = 0; i < 2; i++) {
        for(int j = 0; j < 2; j++) {
            if(c[i][j] < 0) return 0;
        }
    }
    int seg0, seg1;
    if(c[1][0] == c[0][1]) seg1 = c[1][0] + 1, seg0 = c[1][0];
    else seg1 = c[1][0], seg0 = c[1][0];
    int cnt0 = c[0][0] + seg0;
    int cnt1 = c[1][1] + seg1;
    if(cnt0 + cnt1 != n) return 0;
    return 1LL * C(cnt0 - 1, seg0 - 1) * C(cnt1 - 1, seg1 - 1) % mod;
}

int solve(char *s, int n) {
    int ans = 0;
    memcpy(cc, c, sizeof(cc));
    for(int i = 2; i <= n; i++) {
        if(s[i] == '1') {
            cc[s[i - 1] - '0'][0]--;
            ans += getAns0(cc, n - i + 1);
            if(ans >= mod) ans -= mod;
            cc[s[i - 1] - '0'][0]++;
        }
        cc[s[i - 1] - '0'][s[i] - '0']--;
        if(cc[s[i - 1] - '0'][s[i] - '0'] < 0) return ans;
    }
    int ok = 1;
    for(int i = 0; i < 2; i++) {
        for(int j = 0; j < 2; j++) {
            if(cc[i][j]) ok = 0;
        }
    }
    ans = (ans + ok) % mod;
    return ans;
}

int check(char *s, int n) {
    memset(cc, 0, sizeof(cc));
    for(int i = 1; i < n; i++) cc[s[i] - '0'][s[i + 1] - '0']++;
    for(int i = 0; i < 2; i++) {
        for(int j = 0; j < 2; j++) {
            if(cc[i][j] != c[i][j]) return 0;
        }
    }
    return 1;
}

int main() {
    inv[1] = F[0] = Finv[0] = 1;
    for(int i = 2; i < N; i++) inv[i] = 1LL * (mod - mod / i) * inv[mod % i] % mod;
    for(int i = 1; i < N; i++) F[i] = 1LL * F[i - 1] * i % mod;
    for(int i = 1; i < N; i++) Finv[i] = 1LL * Finv[i - 1] * inv[i] % mod;
    scanf("%s%s", s + 1, t + 1);
    int len_s = strlen(s + 1);
    int len_t = strlen(t + 1);
    scanf("%d%d%d%d", &c[0][0], &c[0][1], &c[1][0], &c[1][1]);
    len = 1;
    while(!getAns1(c, len) && len <= len_t) len++;
    if(len > len_t) return puts("0"), 0;
    int ans = 0;
    if(len_s < len && len < len_t) ans = getAns1(c, len);
    else if(len < len_s || len > len_t) ans = 0;
    else if(len == len_s && len == len_t) {
        ans = solve(t, len_t);
        ans -= solve(s, len_s) - check(s, len_s);
        if(ans < 0) ans += mod;
        if(ans >= mod) ans -= mod;
    }
    else if(len == len_s) {
        ans = getAns1(c, len);
        ans -= solve(s, len_s) - check(s, len_s);
        if(ans < 0) ans += mod;
    }
    else if(len == len_t) {
        ans = solve(t, len_t);
    }
    else assert(0);
    printf("%d\n", ans);
    return 0;
}

/**
**/