Find Needle in a Haystack

Given two strings, haystack and needle, find the index of the first occurrence of the needle within the haystack. If not found, return -1.

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Note: Do not use String's substring method. The efficient way would be to make use of Knuth-Morris-Pratt algorithm. The index returned should be zero based.

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Example:

Input : ababcdefgh
Pattern : abcd

Output : 2