Implement strStr()

public class Solution {
    public String strStr(String haystack, String needle) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        int needleLen = needle.length();
        int haystackLen = haystack.length();
        if (needleLen > haystackLen)
            return null;

        if (needleLen == 0)
            return haystack;
            
        int i = 0;
        for (; i < haystackLen - needleLen + 1;) {
            int j = 0;
            for (; j < needleLen;) {
                if (needle.charAt(j) == haystack.charAt(i)) {
                    i++;
                    j++;
                } else {
                    break;
                }
            }
            if (j == needleLen) {
                i = i - j;
                return haystack.substring(i, haystackLen);
            } else {
                i = i - j + 1;
            }

        }

        return null;
    }
}

O(m*n) level 1

KMP algorithm level 5