POJ 1018 Communication System（搜索/贪心/动归）

点击打开链接

搜索（TLE）：

//poj 1018
//2014-6-10
#include <iostream>
#include <cstdio>
using namespace std;
const int MAX_N = 100;
const int INF = 0x3f3f3f3f;
int n;
struct M
{
int b, p;
} m[MAX_N][MAX_N];
int num[MAX_N];

double ans = -1;

//force search
void search ( int cur , int minB, int sum )
{
if ( cur == n )
{
ans = max ( ans, 1.0 * minB / sum );
return;
}

for ( int j = 0; j < num[cur]; j++ )
{
int Min = min ( minB, m[cur][j].b );
search ( cur + 1, Min, sum + m[cur][j].p );
}
}

int main()
{
//freopen ( "in.txt", "r", stdin );
int t;
scanf ( "%d", &t );
while ( t-- )
{
ans = -1;
scanf ( "%d", &n );
for ( int i = 0; i < n; i++ )
{
scanf ( "%d", &num[i] );
for ( int j = 0; j < num[i]; j++ )
{
scanf ( "%d%d", &m[i][j].b, &m[i][j].p );
}
}
search ( 0, INF, 0 );
printf ( "%.3f\n", ans );
}

return 0;
}

贪心法（AC / 47MS)：

从1到Max Bandwidth枚举brandwidth，对于每个manufacturers取最小 price。

int maxb = -1;
void solve()
{
//enmu all bandwidth
for ( int b = 1; b <= maxb; b++ )
{
int sum = 0;
for ( int i = 0; i < n; i++ )
{
int Min = INF;
for ( int j = 0; j < num[i]; j++ )
{
if(m[i][j].b >= b)
Min = min ( Min, m[i][j].p );
}
sum += Min;
}
ans = max ( ans, 1.0 * b / sum );
}
}

动归 AC / 16MS（参照别人的= = ，可见我DP有多渣渣）：

dp[i][j] 选择第i个设备时，带宽为j的最小费用。

#include <iostream>
#include <cstdio>
using namespace std;
const int MAX_N = 100;
const int INF = 0x3f3f3f3f;
int n;
struct M
{
int b, p;
} m[MAX_N][MAX_N];
int num[MAX_N];

int maxb = -1;
int d[MAX_N][1009];
void dp()
{
memset ( d, 0x3f, sizeof ( d ) );
memset ( d, 0, sizeof ( d[0] ) );

for ( int i = 1; i <= n; i++ )
{
for ( int j = 1; j <= num[i]; j++ )
{
for ( int k = 1; k <= m[i][j].b; k++ )
{
d[i][k] = min ( d[i][k], d[i - 1][k] + m[i][j].p );
}
}
}

double ans = 0;
for ( int i = 1; i <= maxb; i++ )
{
ans = max ( ans, 1.0 * i / d
[i] );
}

printf ( "%.3lf\n", ans );
}

int main()
{
freopen ( "in.txt", "r", stdin );
int t;
scanf ( "%d", &t );
while ( t-- )
{
scanf ( "%d", &n );
for ( int i = 1; i <= n; i++ )
{
scanf ( "%d", &num[i] );
for ( int j = 1; j <= num[i]; j++ )
{
scanf ( "%d%d", &m[i][j].b, &m[i][j].p );
maxb = max ( maxb, m[i][j].b );
}
}

dp();
}

return 0;
}