2017 校招真题 网易 数列还原

https://www.nowcoder.com/practice/b698e67a2f5b450a824527e82ed7495d?tpId=85&tqId=29837&tPage=1&rp=1&ru=/ta/2017test&qru=/ta/2017test/question-ranking

数据太弱了，O(n*10!)都能过

简单的暴力，每次交换两个数遍历全排列，统计交换的两个数之间对顺序对个数的影响

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<string>
#include<algorithm>
#include<iostream>
#include<queue>
#include<map>
#include<cmath>
#include<set>
#include<stack>
#define ll long long
#define pb push_back
#define max(x,y) ((x)>(y)?(x):(y))
#define min(x,y) ((x)>(y)?(y):(x))
#define cls(name,x) memset(name,x,sizeof(name))
#define fs first
#define sc second
#define mp make_pair
#define L(x) (1<<x)
#define next Next
#define index Index
using namespace std;
const int inf=1e9+10;
const ll llinf=1e17+10;
const int maxn=1e2+10;
const int maxm=25e2+10;
const int mod=1e9+7;
int n,k;
int num[maxn];
int save[20];
int vis[maxn];
int index=0;
int ans=0;
int solve(int *num,int k1,int k2,int val)
{
    if(k1==k2) return val;
    for(int i=k1+1;i<=k2-1;i++)
    {
        if(num[i]>num[k1]&&num[i]<num[k2])
            val-=2;
        else if(num[i]<num[k1]&&num[i]>num[k2])
            val+=2;
    }
    if(num[k1]<num[k2]) val--;
    else if(num[k1]>num[k2]) val++;
    return val;
}
void dfs(int *num,int st,int val)
{
    if(st==index-1)
    {
        if(val==k)
            ans++;
        return ;
    }
    for(int j=st;j<index;j++)
    {
        int temp=solve(num,save[st],save[j],val);
        swap(num[save[st]],num[save[j]]);
        dfs(num,st+1,temp);
        swap(num[save[st]],num[save[j]]);
    }
}
int main()
{
    //freopen("in.txt","r",stdin);
    while(~scanf("%d %d",&n,&k))
    {
        index=0;
        ans=0;
        cls(vis,0);
        for(int i=1;i<=n;i++)
        {
            scanf("%d",&num[i]);
            if(num[i]==0)
                save[index++]=i;
            vis[num[i]]=1;
        }
        for(int i=1,j=0;i<=n;i++)
            if(vis[i]==0)
            num[save[j++]]=i;
        int val=0;
        for(int i=1;i<=n;i++)
            for(int j=i+1;j<=n;j++)
            if(num[i]<num[j])
            val++;
        dfs(num,0,val);
        printf("%d
",ans);
    }
	return 0;
}