TheAlgorithms · poyea · Mar 1, 2019 · Nov 30, 2018 · Nov 30, 2018 · Nov 30, 2018
diff --git a/data_structures/stacks/balanced_parentheses.py b/data_structures/stacks/balanced_parentheses.py
@@ -1,6 +1,6 @@
 from __future__ import print_function
 from __future__ import absolute_import
-from .Stack import Stack
+from stack import Stack
 
 __author__ = 'Omkar Pathak'
 
@@ -12,8 +12,10 @@ def balanced_parentheses(parentheses):
         if parenthesis == '(':
             stack.push(parenthesis)
         elif parenthesis == ')':
+            if stack.is_empty():
+                return False
             stack.pop()
-    return not stack.is_empty()
+    return stack.is_empty()
 
 
 if __name__ == '__main__':

diff --git a/data_structures/stacks/infix_to_prefix_conversion.py b/data_structures/stacks/infix_to_prefix_conversion.py
@@ -0,0 +1,61 @@
+"""
+Output:
+
+Enter an Infix Equation = a + b ^c
+ Symbol  |  Stack  | Postfix
+----------------------------
+   c     |         | c
+   ^     | ^       | c
+   b     | ^       | cb
+   +     | +       | cb^
+   a     | +       | cb^a
+         |         | cb^a+
+
+	 a+b^c (Infix) ->  +a^bc (Prefix)
+"""
+
+def infix_2_postfix(Infix):
+    Stack = []
+    Postfix = []
+    priority = {'^':3, '*':2, '/':2, '%':2, '+':1, '-':1}       # Priority of each operator
+    print_width = len(Infix) if(len(Infix)>7) else 7
+
+    # Print table header for output
+    print('Symbol'.center(8), 'Stack'.center(print_width), 'Postfix'.center(print_width), sep = " | ")
+    print('-'*(print_width*3+7))
+
+    for x in Infix:
+        if(x.isalpha() or x.isdigit()): Postfix.append(x)       # if x is Alphabet / Digit, add it to Postfix
+        elif(x == '('): Stack.append(x)                         # if x is "(" push to Stack
+        elif(x == ')'):                                         # if x is ")" pop stack until "(" is encountered
+            while(Stack[-1] != '('):
+                Postfix.append( Stack.pop() )                   #Pop stack & add the content to Postfix
+            Stack.pop()
+        else:
+            if(len(Stack)==0): Stack.append(x)                  #If stack is empty, push x to stack
+            else:
+                while( len(Stack) > 0 and priority[x] <= priority[Stack[-1]]):      # while priority of x is not greater than priority of element in the stack
+                    Postfix.append( Stack.pop() )                                   # pop stack & add to Postfix
+                Stack.append(x)                                 # push x to stack
+
+        print(x.center(8), (''.join(Stack)).ljust(print_width), (''.join(Postfix)).ljust(print_width), sep = " | ")         # Output in tabular format
+
+    while(len(Stack) > 0):                                  # while stack is not empty
+        Postfix.append( Stack.pop() )                       # pop stack & add to Postfix
+        print(' '.center(8), (''.join(Stack)).ljust(print_width), (''.join(Postfix)).ljust(print_width), sep = " | ")         # Output in tabular format
+
+    return "".join(Postfix)             # return Postfix as str
+
+def infix_2_prefix(Infix):
+    Infix = list(Infix[::-1])       # reverse the infix equation
+
+    for i in range(len(Infix)):
+        if(Infix[i] == '('): Infix[i] = ')'         # change "(" to ")"
+        elif(Infix[i] == ')'): Infix[i] = '('       # change ")" to "("
+
+    return (infix_2_postfix("".join(Infix)))[::-1]  # call infix_2_postfix on Infix, return reverse of Postfix
+
+if __name__ == "__main__":
+    Infix = input("\nEnter an Infix Equation = ")       #Input an Infix equation
+    Infix = "".join(Infix.split())                      #Remove spaces from the input
+    print("\n\t", Infix, "(Infix) -> ", infix_2_prefix(Infix), "(Prefix)")
diff --git a/data_structures/stacks/postfix_evaluation.py b/data_structures/stacks/postfix_evaluation.py
@@ -0,0 +1,50 @@
+"""
+Output:
+
+Enter a Postfix Equation (space separated) = 5 6 9 * +
+ Symbol  |    Action    | Stack
+-----------------------------------
+       5 | push(5)      | 5
+       6 | push(6)      | 5,6
+       9 | push(9)      | 5,6,9
+         | pop(9)       | 5,6
+         | pop(6)       | 5
+       * | push(6*9)    | 5,54
+         | pop(54)      | 5
+         | pop(5)       |
+       + | push(5+54)   | 59
+
+	Result =  59
+"""
+
+import operator as op
+
+def Solve(Postfix):
+    Stack = []
+    Div = lambda x, y: int(x/y)     # integer division operation
+    Opr = {'^':op.pow, '*':op.mul, '/':Div, '+':op.add, '-':op.sub}     # operators & their respective operation
+
+    # print table header
+    print('Symbol'.center(8), 'Action'.center(12), 'Stack', sep = " | ")
+    print('-'*(30+len(Postfix)))
+
+    for x in Postfix:
+        if( x.isdigit() ):          # if x in digit
+            Stack.append(x)         # append x to stack
+            print(x.rjust(8), ('push('+x+')').ljust(12), ','.join(Stack), sep = " | ")      # output in tabular format
+        else:
+            B = Stack.pop()             # pop stack
+            print("".rjust(8), ('pop('+B+')').ljust(12), ','.join(Stack), sep = " | ")      # output in tabular format
+
+            A = Stack.pop()             # pop stack
+            print("".rjust(8), ('pop('+A+')').ljust(12), ','.join(Stack), sep = " | ")      # output in tabular format
+
+            Stack.append( str(Opr[x](int(A), int(B))) )         # evaluate the 2 values poped from stack & push result to stack
+            print(x.rjust(8), ('push('+A+x+B+')').ljust(12), ','.join(Stack), sep = " | ")      # output in tabular format
+
+    return int(Stack[0])
+
+
+if __name__ == "__main__":
+    Postfix = input("\n\nEnter a Postfix Equation (space separated) = ").split(' ')
+    print("\n\tResult = ", Solve(Postfix))