P3799题解(枚举)

一道不那么暴力的枚举题

P3799

点击查看代码

#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;
int a[10001];
int b[10001];
int mem[10001] = {0};
void quicksort(int l,int r){
	int mid = (l + r) >> 1;
	if(l >= r) return;
	quicksort(l,mid);
	quicksort(mid + 1,r);
	int cnt = l;
	int x = l,y = mid + 1;
	while(x <= mid && y <= r){
		if(a[x] < a[y]){
			b[cnt] = a[x];
			cnt++;
			x++;
		}else{
			b[cnt] = a[y];
			cnt++;
			y++;
		}
	} 
	while(x <= mid){
		b[cnt++] = a[x++];
	}
	while(y <= r){
		b[cnt++] = a[y++];
	}
	for(int i = l;i <= r;i++){
		a[i] = b[i];
	}
}
int main(){
	int n;
	cin >> n;
	int maxn = -0x3f3f3f;
	for(int i = 1;i <= n;i++){
		cin >> a[i];
		mem[a[i]]++;
		maxn = max(maxn,a[i]);
	}
	quicksort(1,n);
	//cout << endl;
	long long ans = 0;
	long long temp;
	for(int i = maxn;i >= 1;i--){
		if(mem[i] >= 2){
			temp = (mem[i]) * (mem[i] - 1) / 2;
			//cout << i <<" "<< temp << endl;
			for(int j = i - 1;j >= ceil((double) i / 2.00);j--){
				if(mem[j] >= 1 && mem[i - j] >= 1 && j != i - j){
					//ans++;
					//ans = (ans + 1) % (long long)(1e9 + 7);
					ans += temp * (mem[i - j]) * (mem[j]);
					//cout << mem[j] <<" "<< mem[i - j] << " " << ans << endl;
					ans %= (long long) (1e9 + 7);
				}
				else if(mem[j] >= 1 && mem[i - j] >= 1 && j == i - j){
					//ans++;
					//ans = (ans + 1) % (long long)(1e9 + 7);
					ans += temp * (mem[j]) * (mem[j] - 1) / 2;
					//cout << mem[j] <<" "<< mem[i - j] << " " << ans << endl;
					ans %= (long long) (1e9 + 7);
				}
			}
		}
	}
	cout << ans << endl;
}