poj 2785

4 Values whose Sum is 0 Time Limit: 15000MS   Memory Limit: 228000K Total Submissions: 18686   Accepted: 5546 Case Time Limit: 5000MS Description The SUM problem can be formulated as follows: given four lists A, B, C, D of integer values, compute how many quadruplet (a, b, c, d ) ∈ A x B x C x D are such that a + b + c + d = 0 . In the following, we assume that all lists have the same size n . Input The first line of the input file contains the size of the lists n (this value can be as large as 4000). We then have n lines containing four integer values (with absolute value as large as 2 28 ) that belong respectively to A, B, C and D . Output For each input file, your program has to write the number quadruplets whose sum is zero. Sample Input 6 -45 22 42 -16 -41 -27 56 30 -36 53 -37 77 -36 30 -75 -46 26 -38 -10 62 -32 -54 -6 45 Sample Output 5 Hint Sample Explanation: Indeed, the sum of the five following quadruplets is zero: (-45, -27, 42, 30), (26, 30, -10, -46), (-32, 22, 56, -46),(-32, 30, -75, 77), (-32, -54, 56, 30). Source Southwestern Europe 2005 注意双向枚举，否则单向处理的话会超时。 #include<cstdio> #include<cstring> #include<algorithm> using namespace std; int n; const int N=5000; int a[N],b[N],c[N],d[N]; int cd[N*N]; int main() { while ( scanf ( "%d" ,&n)!=EOF) { for ( int i=0;i<n;i++) { scanf ( "%d" ,&a[i]); scanf ( "%d" ,&b[i]); scanf ( "%d" ,&c[i]); scanf ( "%d" ,&d[i]); } int cnt=0; for ( int i=0;i<n;i++) { for ( int j=0;j<n;j++) { cd[cnt++]=c[i]+d[j]; } } sort(cd,cd+cnt); long long ans=0; for ( int i=0;i<n;i++) { for ( int j=0;j<n;j++) { int tmp=0-(a[i]+b[j]); // printf("a:%d b:%d num:%d\n",a[i],b[i],upper_bound(cd,cd+cnt,tmp)-lower_bound(cd,cd+cnt,tmp)); ans+=upper_bound(cd,cd+cnt,tmp)-lower_bound(cd,cd+cnt,tmp); } } printf ( "%lld\n" ,ans); } return 0; }