【LeetCode 28_字符串_匹配】Implement strStr()

解法一：Brute-force

 1 int strStr(string haystack, string needle)
 2 {
 3     int m = haystack.size();
 4     int n = needle.size();
 5     if (!n) 
 6         return 0;
 7     for (int i = 0; i < m - n + 1; ++i) {
 8         int k = i, j = 0;
 9         while (j < n) {
10             if (needle[j] == haystack[k]) {
11                 j++;
12                 k++;
13             } else {
14                 break;
15             }
16         }
17         if (j == n)
18             return i;
19     }
20     return -1;
21 }

解法二：KMP

 1 vector<int> GetNext(const string& T)
 2 {
 3     int len = T.size();
 4     vector<int> next(len, 0);
 5     for (int i = 1, k = 0; i < len;) {
 6         if (T[i] == T[k])
 7             next[i++] = ++k;
 8         else if (k)
 9             k = next[k - 1];
10         else
11             next[i++] = 0;
12     }
13     return next;
14 }
15 
16 int strStr(string haystack, string needle)
17 {
18     int m = haystack.size();
19     int n = needle.size();
20     if (n == 0)
21         return 0;
22     
23     vector<int> next = GetNext(needle);
24     for (int i = 0, j = 0; i < m;) {
25         if (haystack[i] == needle[j]) {
26             i++;
27             j++;
28         }
29         if (j == n)
30             return i - n;
31         if (i < m && haystack[i] != needle[j]) {
32             if (j)
33                 j = next[j - 1];
34             else
35                 i++;
36         }
37     }
38     return -1;
39 }