Gym 101061F Fairness【dfs+剪枝】

传送门：Gym 101061F

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Fairness time limit per test2.0 s memory limit per test64 MB inputstandard input outputstandard output

Problem Description
Dwik and his brother Samir both received scholarships from a famous university in India. Their father, Besher, wants to send some money with each of them.
Besher has n coins, the i^th coin has a value of ai. He will distribute these coins between his two sons in n steps. In the i^th step, he chooses whether to give the i^th coin to Dwik or to Samir.
Let xi be the absolute difference between the sum of Dwik’s and Samir’s coins after the ith step. The unfairness factor of a distribution is max({x1, x2, …, xn}). Besher wants to minimize the unfairness factor, can you help him?

Input
The first line of the input consists of a single integer t, the number of test cases. Each test case consists of 2 lines:
The first line contains an integer n (1 ≤ n ≤ 100).
The second line contains n integers a1, a2, …, an (1 ≤ ai ≤ 100).

Output
Print t lines, i^th line containing a single integer, the answer to the i^th test case.

Sample Input
2
5
1 2 1 4 3
7
4 5 6 1 1 3 4

Sample Output
2
5

Note
In the first sample test, besher has 5 coins (1, 2, 1, 4, 3), he can distribute them in the following way:

Step 1: Give the first coin to dwik , d = 1, s = 0 => x1=|1 - 0|=1

Step 2: Give the second coin to samir, d = 1, s = 2 => x2=|1 - 2|=1

Step 3: Give the third coin to samir, d = 1, s = 3 => x3=|1 - 3|=2

Step 4: Give the fourth coin to dwik, d = 5, s = 3 => x4=|5 - 3|=2

Step 5: Give the fifth coin to samir, d = 5, s = 6 => x5=|5 - 6|=1

max({x1, x2, x3, x4, x5}) = 2

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题意：

给定n个权值,按序做n次选择,
每次选择把该权值分配给A或B,
其最终的权值既是在决策过程中A和B差值绝对值的最大值,
我们的目标是使这个函数最小化.输出最小权值.

题解：

当时做的时候想用dp做，但是状态怎么转移我一直想不出来，就看到他的n的范围也不是很大，就想有dfs做。
直接做交上去后超时了。然后就想到了剪枝，每次只要该行的差大于当前最大值了，就直接退出。

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AC代码：

#include<iostream>
#include<algorithm>
#include<cstring>
#include<cstdio>
#define ll long long
#define inf 0x3f3f3f3f
using namespace std;
const int N=110000;
int t;
int n;
int a[110];
int maxn;
void dfs(int x,int m,int s,int d)
{
  if(m>=maxn)
    return ;
  if(x>=n)
  {
    maxn=m;
    return;
  }
  if(abs(s-d)>m) m=abs(s-d);
  dfs(x+1,m,s+a[x],d);
  dfs(x+1,m,s,d+a[x]);
}
int main()
{
  scanf("%d",&t);
  while(t--)
  {
    scanf("%d",&n);
    for(int i=0;i<n;i++)
    {
      scanf("%d",&a[i]);
    }
    maxn=inf;
    dfs(1,0,a[0],0);
    printf("%d\n",maxn);
  }
  return 0;
}