Added Stack ADT

Author: prakhar1989

linear_datastructures/stack-adt.py

@@ -0,0 +1,75 @@
+class Stack(object):
+    """ A simple stack ADT with top as the end of a list """
+    def __init__(self):
+        self.items = []
+
+    def __str__(self):
+        return ("Stack of size: %d" % len(self.items))
+
+    def isEmpty(self):
+        return len(self.items) == 0
+
+    def push(self, item):
+        self.items.append(item)
+
+    def pop(self):
+        return self.items.pop()
+
+    def top(self):
+        if self.isEmpty(): return None
+        return self.items[len(self.items)-1]
+
+def string_reverse(s):
+    stack = Stack()
+    rev = []
+    for c in s: stack.push(c)
+    while not stack.isEmpty():
+        rev.append(stack.pop())
+    return "".join(rev)
+
+def match_paren(parens):
+    """ returns true or false if parenthesis
+    expression passed is matching"""
+    stack = Stack()
+    for b in parens:
+        if b == "(":
+            stack.push(1)
+        else: # b == ")"
+            if not stack.isEmpty():
+                stack.pop()
+            else:
+                return False
+    return stack.isEmpty()
+
+
+def infix_to_postfix(infixexpr):
+    prec = { '+': 2, '-': 2, '*': 3, '/': 3,'(': 1 } # denoting precedence
+    operator_stack = Stack()
+    operators = "+-*/()"
+    output_list = []
+    for token in infixexpr.split():
+        if token not in operators:
+            output_list.append(token)
+        elif token == "(":
+            operator_stack.push("(")
+        elif token == ")":
+            topToken = operator_stack.pop()
+            while topToken != "(":
+                output_list.append(topToken)
+                topToken = operator_stack.pop()
+        else: # an operator
+            while (not operator_stack.isEmpty()) and \
+                (prec[operator_stack.top()] >= prec[token]):
+                output_list.append(operator_stack.pop())
+            operator_stack.push(token)
+
+    # tokens exhausted - empty out the stack
+    while not operator_stack.isEmpty():
+        output_list.append(operator_stack.pop())
+    return " ".join(output_list)
+
+
+if __name__ == "__main__":
+    expr = ["A * B + C * D", "( A + B ) * C - ( D - E ) * ( F + G )"]
+    for e in expr:
+        print infix_to_postfix(e)