[算法] kmp实现

字符串查找是经典场景,也是面试中最常见的一道题。

 

说来惭愧,毕业3年了,才明白了kmp算法的实现,以前一直以为这类算法是基础,工作中中不会碰到【也的确没有碰到过。。。】

但是,对这些基本算法结构的理解是做一个工程师最基本的技能,好好学习,天天向上,在年前项目停止打酱油的日子里,敲了个bf和kmp的实现

 

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
int bf(char *s, char* p) {
    int i, j;
    int lens,lenp;
    if(NULL == s || NULL == p) {
        return -1;    
    }
    lens = strlen(s);
    lenp = strlen(p);
    for(i = 0; i < lens; i++) {
        j = 0;
        while(s[i] == p[j] && j < lenp) {
            i ++;
            j ++;    
        }     
        if(j == lenp) {
            return i - lenp;    
        }
        i = i - j + 1;
    }   
    return -1;
}
 
int main() {
    int ret;
    char s[] = "abcdhelloadf";
    char p[] = "hello";
    ret = bf(s, p);
    printf("%d", ret);
    return 0;
}

  

 

 

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
#define MAXN 100
 
char s[MAXN] = "ababaababcaba";
char p[MAXN] = "ababc";
int next[MAXN];
 
int kmp(char *s, char *p) {
    int i, j;   
    int lens, lenp;
    if( NULL == s || NULL == p) {
        return -1;
    }
    lens = strlen(s);
    lenp = strlen(p);
    for(i = 0; i < lens; i++) {
        j = 0;
        while(s[i] == p[j] && j < lenp) {
            i ++;
            j ++;    
        }     
        if( j == lenp) {
            return i - j;     
        }
        if(next[j] != -1){
            j = next[j];    
        }
        else {
            j = 0;
            i ++;    
        }
    }
    return -1;
}
 
void get_next(char *p, int *next) {
    int i, j;
    int len;
    int tmp;
    len = strlen(p);
    for(i = 0; i < len; i++) {
        if(i == 0) {
            next[i] = -1;    
        }
        else if(i == 1) {
            next[i] = 0;    
        }
        else {
            tmp = i - 1;        
            for(j = tmp; j >= 0; j--) {
                if(equal(p, i, j)) {
                    next[i] = j;
                    break;
                }    
            }
        }
    }
}
 
int equal(char *p, int i, int j) {
    int tmpi;
    for(tmpi = 0; tmpi < j; tmpi++) {
        if(p[tmpi] != p[i, i - j + tmpi]) {
            return 0;    
        }      
    }
    return 1;
}
 
int main() {
    int lenp;
    int ret;
    get_next(p, next);
    ret = kmp(s, p);
    printf("%d\n", ret);
     
    return 0;
}

  

[算法] kmp实现

上一篇:diff patch


下一篇:UML解惑:图说UML中的六大关系