【HDOJ6628】permutation 1（dfs）

题意：求1到n的排列中使得其差分序列的字典序为第k大的原排列

n<=20，k<=1e4

思路：爆搜差分序列，dfs时候用上界和下界剪枝

 #include<bits/stdc++.h>
 using namespace std;
 typedef long long ll;
 typedef unsigned int uint;
 typedef unsigned long long ull;
 typedef pair<int,int> PII;
 typedef pair<ll,ll> Pll;
 typedef vector<int> VI;
 typedef vector<PII> VII;
 #define N  1100000
 #define M  4100000
 #define fi first
 #define se second
 #define MP make_pair
 #define pi acos(-1)
 #define mem(a,b) memset(a,b,sizeof(a))
 #define rep(i,a,b) for(int i=(int)a;i<=(int)b;i++)
 #define per(i,a,b) for(int i=(int)a;i>=(int)b;i--)
 #define lowbit(x) x&(-x)
 #define Rand (rand()*(1<<16)+rand())
 #define id(x) ((x)<=B?(x):m-n/(x)+1)
 #define ls p<<1
 #define rs p<<1|1

 const ll MOD=,inv2=(MOD+)/;
       double eps=1e-;
       int INF=1e9;

 int b[],c[],ans[],d[],n,K;

 int read()
 {
    int v=,f=;
    char c=getchar();
    while(c<||<c) {if(c=='-') f=-; c=getchar();}
    while(<=c&&c<=) v=(v<<)+v+v+c-,c=getchar();
    return v*f;
 }

 void dfs(int k,int l,int r)
 {
     if(l>r) return;
     if(K==) return;
     if(k==n)
     {
         K--;
         if(K==)
         {
             rep(i,,n) ans[i]=c[i]+l;
         }
         return;
     }
     rep(i,-c[k]-n,n-c[k]-)
     {
         int t=c[k]+i+n;
         if(!b[t])
         {
             b[t]=;
             c[k+]=c[k]+i;
             dfs(k+,max(l,-c[k+]+),min(r,n-c[k+]));
             b[t]=;
         }
     }
 }

 int main()
 {
     int cas=read();
     while(cas--)
     {
         n=read(),K=read();
         rep(i,,n+n) b[i]=;
         b[n]=;
         dfs(,,n);
         rep(i,,n-) printf("%d ",ans[i]);
         printf("%d\n",ans[n]);
     }

     return ;
 }