Kolya got string $ s $ for his birthday, the string consists of small English letters. He immediately added $ k $ more characters to the right of the string. Then Borya came and said that the new string contained a tandem repeat of length $ l $ as a substring. How large could $ l $ be? See notes for definition of a tandem repeat.写出c++代码

很高兴为您解答问题！以下是C语言代码：

#include <stdio.h>
#include <string.h>

#define MAXN 100005

int n, k, len;
char s[MAXN], t[MAXN * 2];
int fail[MAXN * 2];

int max(int a, int b) { return a > b ? a : b; }

void get_fail() {
    fail[0] = fail[1] = 0;
    for (int i = 1; i < len; ++i) {
        int j = fail[i];
        while (j && t[i] != t[j]) j = fail[j];
        fail[i + 1] = t[i] == t[j] ? j + 1 : 0;
    }
}

int main() {
    scanf("%s%d", s, &k);
    n = strlen(s), len = n + k;
    for (int i = 0; i < len; ++i) t[i] = i < n ? s[i] : s[i - n];
    get_fail();
    int ans = 0;
    for (int i = 1; i < len; ++i) {
        int cur = fail[i];
        while (cur > n) cur = fail[cur];
        if (cur * 2 >= n) ans = max(ans, cur);
    }
    printf("%d\n", ans);
    return 0;
}

这个问题是一个字符串算法的经典题目，采用 KMP 算法可以在线性时间内解决。