KMP相关算法

KMP算法

KMP算法的关键是利用相同真前后缀的长度来减少搜索的次数，提高效率

未优化版

void GetNext (string P, int next[]) {    int p_len = P.size();    int i = 0;    int j = -1;    next[0] = -1;    while (i < p_len) {        if (j == -1 || P[i] == P[j]) {            i++;            j++;            next[i] = j;        }        else {            j = next[j];        }    }}int KMP (string S, string T, int next[]) {    GetNext (T, next);    int s_len = S.size();    int t_len = T.size();    int i = 0;    int j = 0;    while (j < t_len && i < s_len) {        if (j==-1 || S[i] == T[j]) {            i++;            j++;        }        else {            j = next[j];        }    }    if (j == t_len) {        return i - j;    }    else {        return -1;    }}

优化版

void GetNext (string P, int nextval[]) {    int p_len = P.size();    int i = 0;    int j = -1;    nextval[0] = -1;    while (i < p_len) {        if (j == -1 || P[i] == P[j]) {            i++;            j++;            if (P[i] != P[j]) {                nextval[i] = j;            }            else {                nextval[i] = nextval[j];            }        }        else {            j = nextval[j];        }    }}int KMP (string S, string T, int nextval[]) {    GetNext (T, nextval);    int s_len = S.size();    int t_len = T.size();    int i = 0;    int j = 0;    while (j < t_len && i < s_len) {        if (j==-1 || S[i] == T[j]) {            i++;            j++;        }        else {            j = nextval[j];        }    }    if (j == t_len) {        return i - j;    }    else {        return -1;    }}

扩展KMP算法

void GetNext (char T[], int t_len, int next[]) {    int a = 0, p = 0;    next[0] = t_len;    for(int i = 1; i < t_len; i++) {        if (p <= i || i + next[i - a] >= p) {            if (p <= i) {                p = i;            }            while (p < t_len && T[p] == T[p - i]) {                p++;            }            next[i] = p - i;            a = i;        }        else {            next[i] = next[i - a];        }    }}

参考文献：

[1]：KMP 算法(1)：如何理解 KMP