题目:
In Frobnia, a far-away country, the verdicts in court trials are determined by a jury consisting of members of the general public. Every time a trial is set to begin, a jury has to be selected, which is done as follows. First, several people are drawn randomly from the public. For each person in this pool, defence and prosecution assign a grade from 0 to 20 indicating their preference for this person. 0 means total dislike, 20 on the other hand means that this person is considered ideally suited for the jury.
Based on the grades of the two parties, the judge selects the jury. In order to ensure a fair trial, the tendencies of the jury to favour either defence or prosecution should be as balanced as possible. The jury therefore has to be chosen in a way that is satisfactory to both parties.
We will now make this more precise: given a pool of n potential jurors and two values di (the defence's value) and pi (the prosecution's value) for each potential juror i, you are to select a jury of m persons. If J is a subset of {1,..., n} with m elements, then D(J ) = sum(dk) k belong to J
and P(J) = sum(pk) k belong to J are the total values of this jury for defence and prosecution.
For an optimal jury J , the value |D(J) - P(J)| must be minimal. If there are several jurys with minimal |D(J) - P(J)|, one which maximizes D(J) + P(J) should be selected since the jury should be as ideal as possible for both parties.
You are to write a program that implements this jury selection process and chooses an optimal jury given a set of candidates.
input:
The input file contains several jury selection rounds. Each round starts with a line containing two integers n and m. n is the number of candidates and m the number of jury members.
These values will satisfy 1<=n<=200, 1<=m<=20 and of course m<=n. The following n lines contain the two integers pi and di for i = 1,...,n. A blank line separates each round from the next.
The file ends with a round that has n = m = 0.
output:
For each round output a line containing the number of the jury selection round ('Jury #1', 'Jury #2', etc.).
On the next line print the values D(J ) and P (J ) of your jury as shown below and on another line print the numbers of the m chosen candidates in ascending order. Output a blank before each individual candidate number.
Output an empty line after each test case.
Sample Input:
4 2
1 2
2 3
4 1
6 2
0 0
Sample Output:
Jury #1
Best jury has value 6 for prosecution and value 4 for defence:
2 3
题意:
在遥远的国家佛罗布尼亚，嫌犯是否有罪，须由陪审团决定。陪审团是由法官从公众中挑选的。先随机挑选n个人作为陪审团的候选人，然后再从这n个人中选m人组成陪审团。选m人的办法是： 控方和辩方会根据对候选人的喜欢程度，给所有候选人打分，分值从0到20。为了公平起见，法官选出陪审团的原则是：选出的m个人，必须满足辩方总分和控方总分的差的绝对值最小。如果有多种选择方案的辩方总分和控方总分的之差的绝对值相同，那么选辩控双方总分之和最大的方案即可。
多组输入，每次输入n和m。1<=n<=200,1<=m<=20,m<=n。接下来n行输入的是控方和辩方对改候选人的打分，候选人编号从1开始到n。最后一组数据输入n=m=0结束。
分析:
看成从n个件物品中选取m件物品的01背包问题。用一个sum数组记录控方和辩方分数之和，de数组记录控方和辩方分数之差，并且记录当前阶段所有控方和辩方分数之和与差，这个状态可以用一个dp数组记录，此外dp数组还应记录当前已经选择的成员数，对于当前成员我们可以选择它或者不选它，如果选择它，那么控辩双方的和与差就要加上当前成员的值，并且成员数要增加一个，那么如何判断选择它还是不选它，如果选择了它能使当前控辩双方分差不变的情况下控辩双方的和变大我们就选择它，否则就跳过它，dp[i][j][k]代表当前成员编号为i，已经选择了j位成员，当前控辩双方分差为k的情况下控辩双方分差之和的最大值，由于数组的第一维可以用滚动数组省去空间，dp[i][j][k]可以转变为dp[j][k]，状态转移可以表示为: if(dp[j-1][k] + sum[i] > dp[j][k+de[i]]) dp[j][k+de[i]] = dp[j-1][k] + sum[i]。代表的是如果选了第i位成员那么选择j名成员且控辩差为k+de[i]的情况能获得更优(大)解，那么就选择这位成员。但是由于k不可为负值而de数组中可能存在负值，所以可以定义20×m的点为初始点，因为每个人打分最大为20分，考虑极端情况也就是m个人的de之和会在-20×m到20×m之间，所以定义dp[0][20*m] = 0，为初始点，其他dp值全设置成-1，代表该状态没有访问过，如果一个状态的前一个状态没有访问过，那么便不需要判断状态转移方程直接continue，最后我们从20×m这个点开始向两头扩展，最先到达的已经访问过的状态便是答案（因为此时的控辩双方差是最小的并且是当前控辩双方差的所有情况中控辩双方的和最大的一个），此外还需要用一个vector定义的path数组记录路径，具体看代码操作。
代码:
#include<cstdio>
#include<vector>
#include<cstring>
using namespace std;
const int maxn = 205;
int sum[maxn],de[maxn];
int dp[25][maxn<<2];
vector<int> path[25][maxn<<2];
int main(void){
int n,m,p,d,cnt = 1;
while (scanf("%d%d",&n,&m),n+m){
for (int i = 1; i <= n; i++){
scanf("%d%d",&p,&d);
sum[i] = p+d;
de[i] = p-d;
}
int flag = m*20;
memset(dp,-1,sizeof dp);
dp[0][flag] = 0;
for (int i = 1; i <= n; i++){
for (int j = m; j >= 1; j--){
for (int k = 0; k <= flag*2; k++){
if (k+de[i] < 0 || k+de[i] > flag*2) continue;
if (dp[j-1][k] == -1) continue;
if (dp[j-1][k]+sum[i] > dp[j][k+de[i]]){
dp[j][k+de[i]] = dp[j-1][k] + sum[i];
path[j][k+de[i]] = path[j-1][k];
path[j][k+de[i]].push_back(i);
}
}
}
}
int ans = 0;
while (dp[m][flag-ans] == -1 && dp[m][flag+ans] == -1) ans++;
int tmp = dp[m][flag+ans]>dp[m][flag-ans]?flag+ans:flag-ans;
printf("Jury #%d
Best jury has value %d for prosecution and value %d for defence:
",cnt++,(dp[m][tmp]+tmp-flag)>>1,(dp[m][tmp]-tmp+flag)>>1);
for (int i = 0; i < m; i++){
printf(" %d",path[m][tmp][i]);
}
puts("
");
}
return 0;
}
需要积累与学习之处:
滚动数组 path记录路径 ans = 0之后一系列操作