思路：抽出子串，暴力+結構體排序  n<=60，最多只會有1830種結果

代碼：

#include<iostream>#include<cstring>#include<cstdio>#include<string>#include<algorithm>using namespace std;const int MAXN=2000;struct node{    string str;    int len;    int cnt;    int pos;    node()    {        str="";        len=0;        cnt=0;        pos=0;    }}ans[MAXN];bool cmp(node a, node b){    if(a.cnt>b.cnt)        return true;    else if(a.cnt==b.cnt)    {        if(a.len>b.len)            return true;        else if(a.len==b.len)        {            return a.pos<b.pos;        }        else        {            return false;        }    }    else    {        return false;    }}void slove(int l, string s){    int length=s.length();    int countnumber=0;    for(int i=l;i<=length;i++)    {        for(int j=0;j<=length-i;j++)        {            string ss=s.substr(j,i);            int flag=0;            for(int k=0;k<=countnumber;k++)            {                if(ans[k].str==ss)                {                    ans[k].cnt++;                    flag=1;                    break;                }            }            if(!flag)            {                ans[countnumber].str=ss;                ans[countnumber].len=ss.length();                ans[countnumber].cnt=1;                ans[countnumber].pos=countnumber;                countnumber++;            }        }    }    sort(ans,ans+countnumber+1,cmp);    cout<<ans[0].str<<endl;}int main(){    string s;    int l;    scanf("%d",&l);    cin>>s;    slove(l,s);    return 0;}