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.

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 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:

public int strStr(String haystack, String needle) {
    if(haystack == null) {
        return -1;
    }
    
    if(needle == null) {
        return haystack.length();
    }
    
    if(haystack.length() < needle.length()) {
        return -1;
    }
    
    for(int i = 0; i <= haystack.length() - needle.length(); i++) {
        String tmp = haystack.substring(i, i+needle.length());
        if(tmp.equals(needle)) {
            return i;
        }
    }
    
    return -1;
}