POJ 2785 4 Values whose Sum is 0

枚举左边两个数的和以及右边两个数的和，用二分查找将其配对并计数。Hash的方法晚点再写。

 

/*Accepted 49208K 5188MS C++ 1146B 2012-05-04 23:56:30 */

#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<algorithm>
using namespace std;
#define MAXN 4005

/*int cmp( const void *_p, const void *_q)
{
    int *p = ( int *)_p;
    int *q = ( int *)_q;
    return *p - *q;
}*/

int a[MAXN], b[MAXN], c[MAXN], d[MAXN];
int x[MAXN * MAXN], y[MAXN * MAXN];
int n, m, ans;

int bs( int num)
{
    int left, right, mid, i, cnt;
    left = 0, right = m;
    while( left < right)
    {
        mid = ( left + right) >> 1;
        if( num + y[mid] >= 0)
            right = mid;
        else left = mid + 1;
    }
    for( i = left, cnt = 0; i < m && y[i] + num == 0; i ++, cnt ++);
    return cnt;
}

int main()
{
    while( scanf( "%d", &n) == 1)
    {
        for( int i = 0; i < n; i ++)
            scanf( "%d%d%d%d", &a[i], &b[i], &c[i], &d[i]);
        m = 0;
        for( int i = 0; i < n; i ++)
            for( int j = 0; j < n; j ++)
            {
                x[m] = a[i] + b[j];
                y[m] = c[i] + d[j];
                m ++;
            }
        sort( x, x + m);
        sort( y, y + m);
        //qsort( x, m, sizeof x[0], cmp);
        //qsort( y, m, sizeof y[0], cmp);
        ans = 0;
        for( int i = 0; i < m; i ++)
            ans += bs( x[i]);
        printf( "%d\n", ans);
    }
    return 0;
}

 

用qsort超时了，百思不得其解！