string permutations exercise

Author: dpetrovics

Diff for: README

@@ -5,4 +5,10 @@ Coding exercises, handy practice for technical interview questions. I will put u
 
 All of the exercises are coded in my favorite language, Clojure.
 
-Most of the exercises are taken from "Cracking the Coding Interview, 4th Edition, by Gayle Laakmann.
+Most of the exercises are taken from "Cracking the Coding Interview, 4th Edition" by Gayle Laakmann.
+
+## License
+
+Copyright (C) 2012 David Petrovics
+
+Distributed under the Eclipse Public License, the same as Clojure.

Diff for: README.md

@@ -29,4 +29,13 @@
     (float
      (math/abs (- hr-angle min-angle)))))
 
+(defn word-frequencies
+  ^{:doc "My implementation of the Clojure 'frequency' function. Takes in a sentence (string) and outputs a map with words as keys and their frequency as values."}
+  [sentence]
+  (reduce
+   (fn [res wrd] (assoc res wrd
+                       (inc (get res wrd 0))))
+   {}
+   (re-seq #"\w+" sentence)))
+
 

Diff for: src/coding_exercises/core.clj

@@ -0,0 +1,45 @@
+(ns string-permutations
+  (:require [clojure.string :as str]))
+
+(def problem-statement
+  "Example: Design an algorithm to print all permutations of a string.")
+
+(comment
+  "The algorithm checks to see if the word is of length one, if so,
+  then just return a list with that word. If the word is longer, chop
+  off the last letter, and insert it in every spot in all permutations
+  of the word minus the last letter. It is a recursive solution.")
+
+(defn insert-letter
+  ^{:doc "Returns a collection of each new word produced by inserting the letter in all possible spots in the word."}
+  [word letter]
+  (let [length (count word)]
+    (for [num (range 0 (inc length))]
+      (str (subs word 0 num)
+           letter
+           (subs word num length)))))
+
+(defn word-permutations
+  ^{:doc "Returns a collection of all permutations of the given word."}
+  [s]
+  (if (= (count s) 1)
+    (list s)
+    (let [subword (subs s 0 (dec (count s)))  ;;ex: 'bar', subword = 'ba'
+          letter (str (last (seq s)))]  ;;the last letter, ie 'r' in 'bar'
+      (apply concat
+             (for [word (word-permutations subword)]
+               (insert-letter word letter))))))
+
+(defn factorial
+  ^{:doc "Returns the factorial of a number."}
+  [num]
+  {:pre [(>= num 1)]}
+  (if (= num 1)
+    1
+    (* num (factorial (dec num)))))
+
+(defn num-permutations
+  ^{:doc "Displays the number of permutations for a given word"}
+  [word]
+  (factorial
+   (count word)))