Implement strStr()

Question: https://leetcode.com/problems/implement-strstr/

题目:

Implement strStr().

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

 Solutions:

1. Naive Solution 很容易实现。O(n²) runtime.
2. Rolling Hash.  之前也实现过，O(n) runtime. 可是 hashcode 容易溢出，并且无法保证完全没有 Conflict。 减少 Conflict 可能性，是采用 BigInteger 来记录 hashcode 的值。
3. KMP Tutorial: https://www.youtube.com/watch?v=GTJr8OvyEVQ&index=6&list=PLbLSJnQZCOwMTKzcGHHJi_1DdFDbriKke
   基本思路是寻找 pattern String 的 Prefix Pattern，不难理解
   
   

Naive Solution:

 1 public int strStr(String haystack, String needle) {
 2     if(haystack == null) {
 3         return -1;
 4     }
 5     
 6     if(needle == null) {
 7         return haystack.length();
 8     }
 9     
10     if(haystack.length() < needle.length()) {
11         return -1;
12     }
13     
14     for(int i = 0; i <= haystack.length() - needle.length(); i++) {
15         String tmp = haystack.substring(i, i+needle.length());
16         if(tmp.equals(needle)) {
17             return i;
18         }
19     }
20     
21     return -1;
22 }