Update Decompressor.md

Author: Joseph Luce

real_interview_questions/Google/Decompressor.md

@@ -2,3 +2,70 @@
 Given a compressed string in which a number followed by [] indicate how many times those characters occur, decompress the string
 Eg. : a3[b2[c1[d]]]e will be decompressed as abcdcdbcdcdbcdcde.
 Assume the string is well formed and number will always be followed by a [].
+
+# SOLUTION
+Need to clarify what a2[b2[c]d2[e]]f would decompress to. I would assume to 'abccdeebccdeef'.
+```
+import re
+
+def decompress(compressed_string):
+    tokens = re.split('([\[|\]])', compressed_string)
+    tokens = [token for token in tokens if token != ''] # remove extra spaces
+    if len(tokens) == 0:
+        return ''
+    token_stack = list()
+    result_stack = list()
+    # token_stack.append(tokens[0])
+    curr_token_index = 0
+    while curr_token_index < len(tokens):
+        curr_token = tokens[curr_token_index]
+        if curr_token == '[':
+            curr_token_index += 1
+            next_token = tokens[curr_token_index]
+            token_stack.append(next_token)
+        elif curr_token == ']':
+            next_result = get_next_result(token_stack, result_stack)
+            result_stack.append(next_result)
+        else:
+            while len(token_stack):
+                next_result = get_next_result(token_stack, result_stack)
+                result_stack.append(next_result)
+            token_stack.append(curr_token)
+            result_stack.append('')
+        curr_token_index += 1
+    while len(token_stack):
+        next_result = get_next_result(token_stack, result_stack)
+        result_stack.append(next_result)
+    return ''.join(result_stack)    
+
+def get_next_result(token_stack, result_stack):
+    if len(token_stack):
+        token = token_stack.pop()
+        top_result = ''
+        if len(result_stack):
+            top_result = result_stack.pop()
+        word, n_dups = parse_token(token)
+        new_result = ''
+        for _ in range(n_dups):
+            new_result += top_result
+        new_result = word + new_result
+        return new_result
+    return ''
+
+def parse_token(token):
+    last_alpha_index = -1
+    for letter in token:
+        if not letter.isdigit():
+            last_alpha_index += 1
+    word = token[0:last_alpha_index+1]
+    number = 1
+    if last_alpha_index != len(token)-1:
+        number = int(token[last_alpha_index+1:])
+    return word, number
+
+string = 'a3[b2[c1[d]]]e'
+assert decompress(string) == 'abcdcdbcdcdbcdcde'
+
+# string = 'a2[b2[c]d2[e]]f'
+# assert decompress(string) == 'abccdeebccdeef'
+```