leetcode: 28. Implement strStr()

# Implement strStr().
#
# Returns the index of the first occurrence of needle in x, 
# or -1 if needle is not part of x.

class Solution():
    def strStr(self, x, needle):
        for i in range(len(x)-len(needle)+1):
            if x[i:i+len(needle)] == needle:
                return i
        return -1


if __name__ == "__main__":
    assert Solution().strStr("a", "") == 0
    assert Solution().strStr("a", "a") == 0
    assert Solution().strStr("abababcdab", "ababcdx") == -1
    assert Solution().strStr("hello", "ll") == 2