[LeetCode] Implement strStr()

 1 class Solution {
 2 public:
 3     int strStr(char *haystack, char *needle) {
 4         if (!*needle)
 5             return 0;
 6         int alen = strlen(haystack);
 7         int blen = strlen(needle);
 8         int i = 0, j = 0;
 9         while (i < alen && j < blen) {
10             if (haystack[i] == needle[j]) {
11                 i++;
12                 j++;
13             } else {
14                 i = i - j + 1;
15                 j = 0;
16             }
17         }
18         if (j == blen) {
19             return i - j;
20         } else {
21             return -1;
22         }
23     }
24 };

 1 class Solution {
 2 public:
 3     int strStr(char *haystack, char *needle) {
 4         return kmp(haystack, needle);
 5     }
 6 private:
 7     int kmp(const char *text, const char *pattern) {
 8         int i = 0;
 9         int j = 0;
10         const int n = strlen(text);
11         const int m = strlen(pattern);
12         int *next = (int*)malloc(sizeof(int)* m);
13         compute_prefix(pattern, next);
14 
15         while (i < n && j < m) {
16             if (j == -1 || text[i] == pattern[j]) {
17                 i++;
18                 j++;
19             } else {
20                 j = next[j];
21             }
22         }
23 
24         free(next);
25         if (j == m)
26             return i - j;
27         else
28             return -1;
29     }
30 
31     void compute_prefix(const char *pattern, int next[]) {
32         int i = 0;
33         int j = -1;
34         const int m = strlen(pattern);
35         next[0] = -1;
36 
37         while (i < m - 1) {
38             if (j == -1 || pattern[i] == pattern[j]) {
39                 i++;
40                 j++;
41                 if (pattern[i] != pattern[j])
42                     next[i] = j;
43                 else
44                     next[i] = next[j];
45             }
46             else {
47                 j = next[j];
48             }
49         }
50     }
51 };