KMP

KMP 是经典的用于字符串模式匹配，可以大幅度减少匹配的次数

其核心重点在于 next 数组的获取

// next 数组
void getnext(string s, int n, int *next){
    next[0] = -1;
    int i = 0, j = -1;
    while(i< n){
        if(j == -1 || s[i] == s[j]){
            i++, j++;
            next[i] = j;
        }
        else j = next[j];
    }
} 
// next 数组
void getnext(string s, int n, int *next){
    int i = 0, j = -1;
    next[0] = j;
    while(i< n){
        while(j != -1 && s[j] != s[i]) j = next[j];
        
        i++, j++;
        if(j >= n) next[i] = next[j-1];
        else next[i] = j; 
    }
}

void kmp(string s1, int n, string s2, int m){
    int next[1000005];
    getnext(s1, n, next);
    int i = 0, j = 0;
    int count = 0;
    while(i< m){
        while(j != -1 && s1[j] != s2[i]) j = next[j];
        i++, j++;
        if(j == n){
            count++;
            j = next[j];
        }
    } 
    cout << count << endl; 
}

就一般而言，并不会让你直接用 KMP 对字符串进行匹配， 而是使用其 next 数组用于其他问题的求解