Z algorithm implementation (#77)

* Implemented Z algorithm

* Fixed bugs in implementation and added tests

* Added README explaining z algorithm

src/algorithms/string/z-algorithm/README.md

@@ -0,0 +1,27 @@
+# Z-algorithm
+
+The Z-algorithm finds occurrences of a "word" `W` 
+within a main "text string" `T` in linear time.
+
+Given a string `S` of length `n`, the algorithm produces 
+an array, `Z` where `Z[i]` represents the ongest substring 
+starting from `S[i]` which is also a prefix of `S`. Finding
+`Z` for the string obtained by concatenating the word, `W` 
+with a nonce character, say `$` followed by the text, `T`,
+helps with pattern matching, for if there is some index `i`
+such that `Z[i]` equals the pattern length, then the pattern
+must be present at that point.
+
+While the `Z` array can be computed with two nested loops, the
+following strategy shows how to obtain it in linear time, based 
+on the idea that as we iterate over the letters in the string 
+(index `i` from `1` to `n - 1`), we maintain an interval `[L, R]`
+which is the interval with maximum `R` such that `1 ≤ L ≤ i ≤ R` 
+and `S[L...R]` is a prefix that is also a substring (if no such 
+interval exists, just let `L = R =  - 1`). For `i = 1`, we can 
+simply compute `L` and `R` by comparing `S[0...]` to `S[1...]`.
+
+## Complexity
+
+- **Time:** `O(|W| + |T|)`
+- **Space:** `O(|W|)`