poj 3623 Best Cow Line, Gold

http://poj.org/problem?id=3623

题意：给一个字符序列，要求不断的从原序列的首或尾取一个字符，重新生成一个字符序列使得字典序最小。

思路：一开始以为直接贪心，比较首尾的字符大小就可以了，深入想就知道，当首尾一样的时候，要依靠后一位作判断。因此问题就转成两个字符串的大小比较（首为顺序，尾为反序）。这样先把原字符串反序放到原字符串的后面，对整个串进行后缀排序，最后就可以rank数组贪心答案。

#include<set>
#include<map>
#include<stack>
#include<queue>
#include<cmath>
#include<bitset>
#include<string>
#include<climits>
#include<cstdio>
#include<vector>
#include<utility>
#include<cstdlib>
#include<cstring>
#include<iostream>
#include<algorithm>
#define IN puts("in")
#define OUT puts("out")
#define FR(x) freopen(x,"r",stdin)
#define FW(x) freopen(x,"w",stdout)
#define MSET(x,y) memset(x,y,sizeof(x))
#define ST system("pause")
#define NU(x) (x[0]*1000+x[1]*100+x[2]*10+x[3])
using namespace std;
const int maxn = 62005;
int wn[maxn],wa[maxn],wb[maxn],wv[maxn],a[maxn],sa[maxn],Rank[maxn],height[maxn];
char r[maxn],ans[maxn];

int cmp(int *r,int a,int b,int l){ return r[a]==r[b]&&r[a+l]==r[b+l]; }
void da(int *r,int *sa,int n,int m)
{
    int i,j,p,*x=wa,*y=wb,*t;
    for(i = 0; i < m; ++ i) wn[i] = 0;
    for(i = 0; i < n; ++ i) wn[x[i]=r[i]]++;
    for(i = 1; i < m; ++ i) wn[i] += wn[i-1];
    for(i = n - 1; i >= 0; --i) sa[--wn[x[i]]] = i;
    for(p = 1,j = 1; p < n; j *= 2,m = p)
    {
        for(p = 0,i = n-j; i < n; ++ i) y[p++] =i;
        for(i = 0; i < n; ++ i) if(sa[i]>=j) y[p++] = sa[i] - j;
        for(i = 0; i < m; ++ i) wn[i] = 0;
        for(i = 0; i < n; ++ i) wn[wv[i]=x[y[i]]]++;
        for(i = 1; i < m; ++ i) wn[i] += wn[i-1];
        for(i = n - 1; i >= 0; --i) sa[--wn[wv[i]]] = y[i];
        for(t = x,x = y,y = t,x[sa[0]] = 0,p=1,i=1; i<n; ++i)
            x[sa[i]]=cmp(y,sa[i-1],sa[i],j)?p-1:p++;
    }
}
void calheight(int *r,int *sa,int n)
{
    int i,j,k = 0;
    for(i = 1; i <= n; ++ i){ Rank[sa[i]] = i; height[i] = 0;}
    for(i = 0; i < n; height[Rank[i++]]=k)
        for(k?k--:0,j=sa[Rank[i]-1]; r[i+k]==r[j+k];++k);
}
int main()
{
        int i,j,k,n;
        char str[2];
        scanf("%d",&n);
        for(i = 0; i < n; ++ i){scanf("%s",str); r[i]=str[0]; }
        r[n] = '#';
        for(i = n + 1; i <= n + n; ++ i) r[i] = r[n+n-i];
        for(i = 0; i <= n + n + 1; ++ i) a[i] = static_cast<int>(r[i]);
        da(a,sa,n+n+2,256);
        calheight(a,sa,n+n+1);
        i = 0; j = n + 1; k = 0;
        while(k<n){
                if(Rank[i]<Rank[j])ans[k++] = r[i++];
                else ans[k++] = r[j++];
        }
        k = 1;
        for(i = 0; i < n; ++ i){
                putchar(ans[i]);
                if(k%80==0)puts("");
                k++;
        }
        return 0;
}