lintcode strStrII

这是题目的介绍：
Implement strStr function in O(n + m) time.

strStr return the first index of the target string in a source string. The length of the target string is m and the length of the source string is n.
If target does not exist in source, just return -1.

看了九章的答案感觉还是没有看懂，地里有没有大神有更易懂的解法啊，谢谢！
（http://www.lintcode.com/en/problem/strstr-ii/）