KMP算法

写在前面：

        模式串下标从0开始，next数组和nextval数组从-1开始;

        例如：

                            abcabd        aaaaaaab

         next          -100012        -10123456

         nextval     -100002        -10000006

代码：

        普通KMP (时间复杂度 O(n + m))

#include <stdio.h>
#include <string.h>
#define N 100

void Get_Next(char t[], int len);
int KMP(char s[], char t[], int lens, int lent);

int next[N];

int main()
{
	char s[N], t[N];
	scanf("%s%s", s, t);
	Get_Next(t, strlen(t));
	int start = KMP(s, t, strlen(s), strlen(t));
	printf("%d\n", start);

	return 0;
}

int KMP(char s[], char t[], int lens, int lent)
{
	int i = 0, j = 0;
	while(i < lens && j < lent){
		if(j == 0 || s[i] == t[j]){               //虽然next是从-1开始，但这儿j最小为0
			i ++;
			j ++;
		}
		else{
			j = next[j];
		}
	}

	if(j == lent) return i - lent;
	else return -1;
}

void Get_Next(char t[], int len)
{
	int i = 0, j = -1;
    next[0] = -1;
    while(i < len){
        if(j == -1 || t[i] == t[j]){
			/* j == -1 表示当前next[i]保持初始值(0)不变,然后next[i+1] = 0; 
			next[i] = j 指 t[i-j+1...i-1] == t[0...j];					*/
            i ++;
            j ++;
            next[i] = j;
        }
        else
			j = next[j];
    }
}

        优化的KMP (时间复杂度 O(m))

#include <stdio.h>
#include <string.h>
#define N 100

void Get_NextVal(char t[], int len);
int KMP(char s[], char t[], int lens, int lent);

int nextval[N];

int main()
{
	char s[N], t[N];
	scanf("%s%s", s, t);
	Get_NextVal(t, strlen(t));
	int start = KMP(s, t, strlen(s), strlen(t));
	printf("%d\n", start);

	return 0;
}

int KMP(char s[], char t[], int lens, int lent)
{
	int i = 0, j = 0;
	while(i < lens && j < lent){
		if(j == 0 || s[i] == t[j]){
			i ++;
			j ++;
		}
		else{
			j = nextval[j];
		}
	}

	if(j == lent) return i - lent;
	else return -1;
}

void Get_NextVal(char t[], int len)
{
	int i = 0, j = -1;
	nextval[0] = -1;
	while(i < len){
		if(j == -1 || t[i] == t[j]){
			i ++;
			j ++;
			if(t[i] != t[j]) nextval[i] = j;
			else nextval[i] = nextval[j];
		}
		else
			j = nextval[j];
	}
}