根據《演算法導論》上的虛擬碼修改，書上的字串索引從1開始，需要修改為從0開始

 1 class Solution {
 2 public:
 3     int strStr(string haystack, string needle) {
 4         int sz1 = haystack.size(), sz2 = needle.size();
 5         if (sz2 == 0) {
 6             return 0;
 7         }
 8         if (sz1 == 0) {
 9             return -1;
10         }
 
11         int q = -1;
12         vector<int>trans = _helper(needle);
13         for (int i = 0; i < sz1; ++i) {
14             while (q > -1 && haystack[i] != needle[q+1]) {
15                 q = trans[q];
16             }
17             if (haystack[i] == needle[q+1]) {
18                 ++q;
 
19             }
20             if (q == sz2-1) {
21                 return i - sz2+1;
22             }
23         }
24         return -1;
25     }
26 private:
27     vector<int> _helper(string needle) {
28         int sz = needle.size();
29         vector<int>res(sz, 0);
30         int k = -1
 
;
31         res[0] = -1;
32         //q表示狀態
33         for (int q = 1; q < sz; ++q) {
34             while (k > -1 && needle[k+1] != needle[q]) {
35                 k = res[k];
36             }
37             if (needle[q] == needle[k+1]) {
38                 ++k;
39             }
40             res[q] = k;
41         }
42         return res;
43     }
44 };