Leetcode-Implement strStr()

Implement strStr().

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

Update (2014-11-02):
The signature of the function had been updated to return the index instead of the pointer. If you still see your function signature returns a char * or String, please click the reload button to reset your code definition.

Solution:

public class Solution {
    public int strStr(String haystack, String needle) {
        if (haystack.length()<needle.length()) return -1;
        if (needle.length()==0) return 0;
        int len1 = haystack.length();
        int len2 = needle.length();
        for (int i=0;i<=len1-len2;i++)
            if (compare(haystack,i,needle))
               return i;

        return -1;
    }

    public boolean compare(String haystack, int start, String needle){
        for (int i=0;i<needle.length();i++){ 
            char c1 = haystack.charAt(start+i);
            char c2 = needle.charAt(i);
            if (c1!=c2) return false;
        }
        return true;
   }

        
}

NOTE: Seems that we can use KMP algorithm. PRACTICE IT!