Many Equal Substrings-（KMP）-Codeforces Round #506 (Div. 3) A题

思路：
利用KMP算法中的next数组，求循环节。
具体过程看代码。

AC code:
#include
#include
#include
#include
#include

using namespace std;
const int maxn = 1e5 + 10;
int nex[maxn];
string str1,str2;
int n = 0,k = 0;
int ans = 0;
void make_nex(){
    int j = 0,k = -1;
    nex[0] = -1;
    int len2 = str2.size();
    while(j < len2){//是j < m而不是m-1,虽然j 
        if(k == -1 || str2[k] == str2[j]){
            if(str2[k+1] == str2[j+1]){
                nex[j+1] = nex[k+1];
                ++j,++k;
            }else{
                nex[j+1] = k+1;
                ++j,++k;
            }
        }else{
            k = nex[k];
        }
    }   
}

int main(){

    while(cin >> n >> k){
        cin >> str2;
        memset(nex,0,sizeof(nex));
        make_nex();
        str1 = str2;

        for(int j = 0;j < k - 1;++j){
            for(int i = nex[n];i < n;++i){
                str1 += str2[i];
            }
        }
        cout << str1 << endl;
    }
    return 0;
}