题目链接：
codeforces 415E
题目大意：
给出n个盒子，每个盒子有朵花，每个盒子的花颜色相同，，不同盒子的花颜色不同，问有多少种方案能恰巧选出s朵花。
题目分析：然后直接容斥做就可以了。 AC代码：;LL;int n;LL s,f[25],inv[25];const LL mod = 1e9+7;LL Inv ( LL num , LL x ){LL ret = 1;while ( x ){if ( x&1 ){ret *= num;ret %= mod;}num *= num;num %= mod;x >>= 1;}return ret;}void init ( ){for ( int i = 1 ; i < 21 ; i++ )inv[i] = Inv( i , mod-2 );}LL com ( LL n , LL m ){if ( n < m ) return 0LL;if ( n== m ) return 1LL;LL ret = 1;for ( LL i = n; i >= (n-m+1) ; i– ){ret *= inv[n+1-i];ret %= mod;ret *= i%mod;ret %= mod;}return ret;}int main ( ){init ( );while (~scanf ( “%d%lld” , &n , &s ) ){for ( int i = 0 ; i < n ; i++ )scanf ( “%lld” , &f[i] );int total = 1<<n;LL ans = com ( s+n-1 , n-1 );for ( int i = 1 ; i < total ; i++ ){int cnt = 0;LL x = 0;for ( int j = 0 ; j < n ; j++ )if ( (1<<j)&i ){x += f[j]+1LL;cnt++;}if ( cnt&1 ){ans -= com ( s+n-1-x , n-1 );ans = (ans%mod+mod)%mod;}else{ans += com ( s+n-1-x , n-1 );ans %= mod;}}printf ( “%lld\n” , ans );}}
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