KMP算法

具体的参照该网址：

https://www.cnblogs.com/zhangtianq/p/5839909.html

实现如下：不过解释 或者说怎么实现的在上面网址

#include<cstdio>
#include<cstring>
#define N 10

int next[N] = {0};

void getNext(char P[]);

int main(void) {
    char T[] = "abcbabcd";
    char P[] = "abc";
    int i = 0, j = 0;//i为主串的指针位置，j为模式串指针位置
    int len_P = strlen(P);
    int len_T = strlen(T);

    getNext(P);
    while (i < len_T && j < len_P) {
        if (j == -1 || T[i] == P[j]) { i++; j++; }
        else j = next[j];
    }

    if (j == len_P) printf("%d", i - j);
    else printf("-1");
}

void getNext(char P[]) {
    next[0] = -1;
    
    int j = 0, k = -1;
    int len = strlen(P);
//p[k]是前缀，p[j]是后缀
    /*
    while (j < len - 1) {
        if (k == -1 || P[j] == P[k]) next[++j] = ++k;
        else k = next[k];
    }//其实这个循环还是可以优化 如下:
    */
    while (j < len - 1) {
        if (k == -1 || P[j] == P[k]) {
            if (P[++j] == P[++k]) next[j] = next[k];
            else next[j] = k;
        }
        else k = next[k];
    }

}