排排站（codevs1247)-哈希练习 —— By litble

#include<algorithm>
#include<iostream>
#include<cstring>
#include<climits>
#include<cstdio>
using namespace std;
const int N=100005;
int n,m,ans,mod=999997,tot;
int a[N][32],b[N][32],bin[32];
int h[1000000],ne[N],q[N];
int ok(int x,int y){
    for(int i=1;i<=m;++i)
        if(b[x][i]!=b[y][i])return 0;
    return 1;
}
void add(int x,int wz){
    for(int i=h[x];i!=-1;i=ne[i])
        if(ok(wz,q[i])){ans=max(ans,wz-q[i]);return;}
    ++tot,q[tot]=wz,ne[tot]=h[x],h[x]=tot;
}
int main()
{
    int i,j,x,kl;
    memset(h,-1,sizeof(h));
    add(0,0);
    scanf("%d%d",&n,&m);
    bin[1]=1;for(i=2;i<=m;++i)bin[i]=bin[i-1]<<1;
    for(i=1;i<=n;++i){
        scanf("%d",&x);
        kl=0;
        for(j=1;j<=m;++j){
            if((x&bin[j]))a[i][j]=1;
            a[i][j]=a[i][j]+a[i-1][j];
        }
        for(j=1;j<=m;++j){
            b[i][j]=a[i][j]-a[i][1];
            kl=kl%mod+b[i][j]<<2;
        }
        if(kl<0)kl=-kl;
        add(kl%mod,i);
    }
    printf("%d",ans);
    return 0;
}