Implement strStr()

Problem

For a given source string and a target string, you should output the first index (from 0) of target string in source string.

If target does not exist in source, just return -1.

Example

If source = "source" and target = "target", return -1.

If source = "abcdabcdefg" and target = "bcd", return 1.

Challenge

O(n^2) is acceptable. Can you implement an O(n) algorithm? (hint: KMP)