Implement strStr() Easy
Implement strStr().
Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.
public int strStr(String haystack, String needle) { if (haystack == null || needle == null) return -1; int m = haystack.length(); int n = needle.length(); Set<String> ns = new HashSet<String>() {{ add(needle); }}; for (int i = 0; i < m - n + 1; ++i) { String cur = haystack.substring(i, i + n); if (ns.contains(cur)) return i; } return -1; }思路：取得haystack中每一个与needle等长的子串，取Hash与needle相比，然后再计算是否相等。 O(n)
