KMP 算法(上篇)

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/* * KMP 算法 * 要点如下： *      1 如何构建next表 *      2 如何利用next表进行模式匹配 *          指针位置的控制问题 */#include <cstdio>#include <cstring>void get_next(char *str, int *next) {    size_t len = strlen(str);    memset(next, 0, len * sizeof(int));    if (len==0) return;    next[0] = -1;    char *p = str+1;    while (*p != '\0') {        char *q = str + next[p-1-str];        while (q >= str) {            if (*(p-1)==*q) {                next[p-str] = next[q-str] + 1;                break;            }            q = str + next[q-str];        }        if (q < str) {            next[p-str] = next[0] + 1;        }        ++p;    }    next[0] = 0;}char *KMP_strstr(char *str1, char *str2) {    int *next = new int[strlen(str2)];    get_next(str2, next);    int index = 0;    char *p = str1;    char *q = str1;    while (*q != '\0') {        char *s = str2 + index;        while (*s==*q && *s!='\0')            ++s, ++q;        if (*s=='\0') {            break;        }        // 控制 s 的位置        index = next[s-str2];        // 控制 q 的位置        if (s==str2) ++q;        // 控制 p 的位置        p = q - index;    }    delete [] next;    return p;}int main() {    char *str1 = "ababaaababababcaaaa";    char str[] = "ababababca";    char *pch = KMP_strstr(str1, str);    printf("%d\n", pch-str1+1);    return 0;}

/* * KMP 算法 * 要点如下： *      1 如何构建next表 *      2 如何利用next表进行模式匹配 *          指针位置的控制问题 */#include <cstdio>#include <cstring>void get_next(char *str, int *next) {    next[0] = -1;    char *p = str+1;    while (*p != '\0') {        char *q = str + next[p-1-str];        while (q >= str) {            if (*(p-1) == *q) {                next[p-str] = next[q-str] + 1;                break;            }            q = str + next[q-str];        }        if (q < str) {            next[p-str] = next[0] + 1;        }        ++p;    }}char *KMP_strstr(char *str1, char *str2) {    int *next = new int[strlen(str2)];    get_next(str2, next);    char *p = str1;    char *q = str2;    while (*p != '\0' && *q != '\0') {        if (q < str2 || *p==*q) ++p, ++q;        else q = str2 + next[q-str2];    }    delete [] next;    if (*q == '\0') return p - (q - str2);    return nullptr;}int main() {    char *str1 = "ababaaababababcaaaa";    char str[] = "ababababca";    char *pch = KMP_strstr(str1, str);    printf("%d\n", pch-str1+1);    return 0;}

/* * KMP 算法 * 要点如下： *      1 如何构建next表 *      2 如何利用next表进行模式匹配 *          指针位置的控制问题 *  这个是精炼的算法，不好理解 */#include <cstdio>#include <cstring>void get_next(char *str, int *next) {    next[0] = -1;    char *p = str;    char *q = str-1;    while (*(p+1) != '\0') {        if (q<str || *p==*q)  {            ++q, ++p;            if (*p != *q) next[p-str] = q-str;            else next[p-str] = next[q-str];        }        else q = str + next[q-str];    }}char *KMP_strstr(char *str1, char *str2) {    int *next = new int[strlen(str2)];    get_next(str2, next);    char *p = str1;    char *q = str2;    while (*p != '\0' && *q != '\0') {        if (q < str2 || *p==*q) ++p, ++q;        else q = str2 + next[q-str2];    }    delete [] next;    if (*q == '\0') return p - (q - str2);    return nullptr;}int main() {    char *str1 = "ababaaababababcaaaa";    char str[] = "ababababca";    char *pch = KMP_strstr(str1, str);    printf("%d\n", pch-str1+1);    return 0;}

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