[组合] Codeforces #571A. Lengthening Sticks

一开始正着做很复杂，然后发现补集转化一下就很方便了，随便搞一下…

#include<cstdio>#include<algorithm>using namespace std;const int maxn=100005;typedef long long LL;int a,b,c,L,d;LL ans;LL get(int a,int b,int c){    int x=min(L+a-d,d-b-c);    return x<0?0:(LL)(x+2)*(x+1)/2;}int main(){    freopen("cf571A.in","r",stdin);    freopen("cf571A.out","w",stdout);    scanf("%d%d%d%d",&a,&b,&c,&L); int _max=max(a,max(b,c));    ans=(LL)(L+3)*(L+2)/2*(L+1)/3;     for(d=_max;d<=_max+L;d++){        ans-=get(a,b,c)+get(b,a,c)+get(c,a,b);        //printf("%d: %d\n",d,get(a,b,c)+get(b,a,c)+get(c,a,b));    }     printf("%I64d\n",ans);    return 0;}