Commit df1a4a5

@@ -0,0 +1,69 @@
+<<-DOC
+Given an encoded string, return its corresponding decoded string.
+
+The encoding rule is: k[encoded_string],
+where the encoded_string inside the square brackets is repeated exactly k times. 
+Note: k is guaranteed to be a positive integer.
+
+Note that your solution should have linear complexity because this is what you will be asked during an interview.
+
+Example
+
+For s = "4[ab]", the output should be
+
+decodeString(s) = "abababab";
+
+For s = "2[b3[a]]", the output should be
+
+decodeString(s) = "baaabaaa";
+
+For s = "z1[y]zzz2[abc]", the output should be
+
+decodeString(s) = "zyzzzabcabc".
+
+Input/Output
+
+[time limit] 4000ms (rb)
+[input] string s
+
+A string encoded as described above. It is guaranteed that:
+
+the string consists only of digits, square brackets and lowercase English letters;
+the square brackets form a regular bracket sequence;
+all digits that appear in the string represent the number of times the content in the brackets that follow them repeats, i.e. k in the description above;
+there are at most 20 pairs of square brackets in the given string.
+Guaranteed constraints:
+
+0 ≤ s.length ≤ 80.
+
+[output] string
+DOC
+
+describe "#decode_string" do
+  def decode_string(message)
+    message
+  end
+
+  [
+    { s: "4[ab]", x: "abababab" },
+    { s: "2[b3[a]]", x: "baaabaaa" },
+    { s: "z1[y]zzz2[abc]", x: "zyzzzabcabc" },
+    { s: "3[a]2[bc]", x: "aaabcbc" },
+    { s: "3[a2[c]]", x: "accaccacc" },
+    { s: "2[abc]3[cd]ef", x: "abcabccdcdcdef" },
+    { s: "", x: nil },
+    { s: "codefights", x: "codefights" },
+    { s: "2[codefights]", x: "codefightscodefights" },
+    { s: "100[codefights]", x: "codefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefightscodefights" },
+    { s: "sd2[f2[e]g]i", x: "sdfeegfeegi" },
+    { s: "2[a]", x: "aa" },
+    { s: "2[a]3[b]4[c]5[d]", x: "aabbbccccddddd" },
+    { s: "2[2[b]]", x: "bbbb" },
+    { s: "2[2[2[b]]]", x: "bbbbbbbb" },
+    { s: "2[2[2[2[b]]]]", x: "bbbbbbbbbbbbbbbb" },
+  ].each do |x|
+    it do
+      expect(decode_string(x[:s])).to eql(x[:x])
+    end
+  end
+end