SPOJ4580 ABCDEF(meet in the middle)

题意

题目链接

Sol

发现abcdef是互不相关的

那么meet in the middle一下。先算出abc的，再算def的

注意d = 0的时候不合法(害我wa了两发。。)

#include<bits/stdc++.h>
#define LL long long 
using namespace std;
const int MAXN = 101, SS = 2e6 + 10;
map<LL, LL> mp;
int N;
LL a[MAXN], ans;
int a1[SS], c1, a2[SS], c2, cnt[SS];
int main() {
//	ios::sync_with_stdio(false);
	cin >> N;
	for(int i = 1; i <= N; i++) cin >> a[i];
	sort(a + 1, a + N + 1);
	for(int i = 1; i <= N; i++) 
		for(int j = 1; j <= N; j++) 
			for(int k = 1; k <= N; k++) 
				a1[++c1] = a[i] * a[j] + a[k];
	for(int i = 1; i <= N; i++) 
		for(int j = 1; j <= N; j++) 
			for(int k = 1; k <= N; k++) 
				if(a[i] != 0) a2[++c2] = a[i] * (a[j] + a[k]);
	sort(a1 + 1, a1 + c1 + 1);
	sort(a2 + 1, a2 + c2 + 1);
	for(int i = 1, j = 1; i <= c2; i++) {
		if(i != 1 && (a2[i] == a2[i - 1])) {cnt[i] = cnt[i - 1]; continue;}
		while(a1[j] <= a2[i] && j <= c1) {
			if(a1[j] == a2[i]) cnt[i]++;
			j++;
		}
	}
	for(int i = 1; i <= c2; i++) ans += cnt[i];
	cout << ans;
	return 0;
}