Create compute_string.md

Author: Joseph Luce

real_interview_questions/Google/compute_string.md

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+# SOLUTION (Run: O(n), Space: O(n), N = number of characters in string)
+This was one given during an onsite interview at google. I did this question with recursion which I believe was a mistake. 
+Before the interview, he said he was testing my ability to write in Python, but the recursion method wasn't a good way to show that.
+Now this solution is the dirty way to do, there is a cleaner pythonic method to this down below.
+
+Overall, this is a medium question, as long as you can see that stacks need to be used in one way or the other, the question is fairly straight forward.
+
+```
+def calculate_string(input):
+    num_stack, op_stack = list([0]), list(['+'])
+    curr_index = 0
+    while curr_index < len(input):
+        #print(num_stack, op_stack)
+        ch = input[curr_index]
+        if ch.isnumeric():
+            operator = op_stack.pop()
+            num1 = num_stack.pop()
+            num2 = int(ch)
+            result = get_result(num1, num2, operator)
+            num_stack.append(result)
+        elif ch == '[':
+            curr_index += 1
+            num = int(input[curr_index])
+            num_stack.append(num)
+        elif ch == ']':
+            num2 = num_stack.pop()
+            num1 = num_stack.pop()
+            operator = op_stack.pop()
+            result = get_result(num1, num2, operator)
+            num_stack.append(result)
+        else: # operator
+            op_stack.append(ch)
+        curr_index += 1
+    return sum(num_stack)
+    
+def get_result(n1, n2, operator):
+    if operator == '+':
+        return n1 + n2
+    elif operator == '-':
+        return n1 - n2
+
+assert calculate_string('1+2-[3+4]') == -4
+assert calculate_string('[1-2]+[3+4]') == 6
+assert calculate_string('1+2-3-4') == -4
+assert calculate_string('[1-2]+3-4') == -2
+assert calculate_string('[1-2]+3-4-[5-6]') == -1
+assert calculate_string('[1]-[2]+[3]-[4]') == -2
+assert calculate_string('-1') == -1
+assert calculate_string('-1+2') == 1
+```
+
+# SOLUTION 2 (Clean Pythonic Way)
+Making use of a dictionary as a way to switch between cases, will greatly simplify the code.
+'''
+
+'''