【BZOJ2886】最短路【组合数】

【题目链接】

同【BZOJ3260的题解】

/* Pigonometry */#include <cstdio>#include <algorithm>using namespace std;typedef unsigned long long ULL;typedef long long LL;const ULL p = 1000000007;void exgcd(ULL a, ULL b, LL &x, LL &y) {b ? (exgcd(b, a % b, y, x), y -= a / b * x) : (x = 1, y = 0);}ULL inv(ULL a) {LL x, y;exgcd(a, p, x, y);if(x < 0) x += p;return x % p;}int main() {ULL n, m; scanf("%llu%llu", &n, &m);if(m > n) swap(n, m); n += m + 1; n %= p;ULL a = 1, b = 1;for(int i = 1; i <= m; i++) {a = (n - i + 1) * a % p;b = i * b % p;}ULL ans = (a * inv(b) % p + n - m - 1) % p;printf("%llu\n", ans);return 0;}