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

解法一：Brute-force

 int strStr(string haystack, string needle)
 {
     int m = haystack.size();
     int n = needle.size();
     if (!n)
         return ;
     for (int i = ; i < m - n + ; ++i) {
         int k = i, j = ;
         while (j < n) {
             if (needle[j] == haystack[k]) {
                 j++;
                 k++;
             } else {
                 break;
             }
         }
         if (j == n)
             return i;
     }
     return -;
 }

解法二：KMP

 vector<int> GetNext(const string& T)
 {
     int len = T.size();
     vector<int> next(len, );
     for (int i = , k = ; i < len;) {
         if (T[i] == T[k])
             next[i++] = ++k;
         else if (k)
             k = next[k - ];
         else
             next[i++] = ;
     }
     return next;
 }

 int strStr(string haystack, string needle)
 {
     int m = haystack.size();
     int n = needle.size();
     if (n == )
         return ;

     vector<int> next = GetNext(needle);
     for (int i = , j = ; i < m;) {
         if (haystack[i] == needle[j]) {
             i++;
             j++;
         }
         if (j == n)
             return i - n;
         if (i < m && haystack[i] != needle[j]) {
             if (j)
                 j = next[j - ];
             else
                 i++;
         }
     }
     return -;
 }