028. Implement strStr()

 1 class Solution {
 2 public:
 3     int strStr(string haystack, string needle) {
 4         if (needle.size() == 0) return 0;
 5         else {
 6             if (haystack.size() == 0) return -1;
 7             else {
 8                 for (int i = 0; i <= static_cast<int>(haystack.size()) - static_cast<int>(needle.size()); ++i) {
 9                     bool flag = true;
10                     for (int j = 0; j < needle.size(); ++j) {
11                         if (haystack[i + j] != needle[j]) {
12                             flag = false; break;
13                         }
14                     }
15                     if (flag) {
16                         return i;
17                     }
18                 }
19                 return -1;
20             }
21         }
22     }
23 };