Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Example 1:

Input: haystack = "hello", needle = "ll"
Output: 2

Example 2:

Input: haystack = "aaaaa", needle = "bba"
Output: -1

Clarification:

What should we return when needle is an empty string? This is a great question to ask during an interview.

For the purpose of this problem, we will return 0 when needle is an empty string. This is consistent to C's strstr() and Java's indexOf().

這道題目顯然是要考慮使用KMP演算法的，那麼現在就來寫一個比較簡單的KMP演算法。

class Solution { public:      int strStr(string haystack, string needle) {         int m = haystack.length(), n = needle.length();         if (!n) {//如果子字串為空             return 0;         }         //kmp         for (int i = 0; i < m - n + 1; i++) {             int j = 0;             for (; j < n; j++) {                 if (haystack[i + j] != needle[j]) {                     break;                 }             }             if (j == n) {                 return i;             }         }         return -1;     } };