Implement strStr().
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
strstr()函数返回匹配的首字符索引。
可以使用常规的匹配算法，蛮力算法
遍历haystack到m-n+1的同时遍历needle从0到n，while来判断两个string对应字符是否相等。
class Solution { public: int strStr(string haystack, string needle) { int m = haystack.size(), n = needle.size(); if (n == 0) return 0; for (int i = 0; i < m - n + 1; i++) { int j = 0; while (haystack[i + j] == needle[j]) { j++; if (j == n) return i; } j++; } return -1; } }; // 6 ms