Added #43

Author: subsr97

challenges/max-stack-implementation.py

@@ -0,0 +1,116 @@
+"""
+#43
+Amazon
+
+Implement a stack that has the following methods:
+    ->  push(val), which pushes an element onto the stack
+    
+    ->  pop(), which pops off and returns the topmost element of the stack. 
+        If there are no elements in the stack, then it should throw an error or return null.
+    
+    ->  max(), which returns the maximum value in the stack currently. 
+        If there are no elements in the stack, then it should throw an error or return null.
+Each method should run in constant time.
+"""
+
+# Node represents a single stack element
+class Node:
+    def __init__(self, val):
+        self.val = val
+        self.prev = None
+    
+    def __str__(self):
+        return "<" + str(self.val)  + ">"
+
+# Ordinary stack implementation with linked-list
+class Stack:
+    def __init__(self):
+        self.top = Node(None)
+    
+    def __str__(self):
+        stringRepresentation = "< "
+        currentNode = self.top
+        while currentNode.val != None:
+            stringRepresentation += str(currentNode)
+            currentNode = currentNode.prev
+        stringRepresentation += " >"
+        return stringRepresentation
+    
+    def push(self, val):
+        newNode = Node(val)
+        newNode.prev = self.top
+        self.top = newNode
+    
+    def pop(self):
+        temp = self.top
+        if temp.val != None:
+            self.top = self.top.prev
+        return temp
+    
+    def peek(self):
+        return self.top.val
+
+# Special Stack inherits Stack and has additional features like max()
+class SpecialStack(Stack):
+    def __init__(self):
+        Stack.__init__(self)
+        self.maxStack = Stack() # maxStack is an ordinary stack that is used to hold the max values.
+    
+    """
+    push(x)
+        1) Push x into Special Stack
+        2) Compare x with the maxStack's top (y)
+            ->  If x > y, push x onto maxStack
+            ->  If y > x, push y onto maxStack
+    """
+    def push(self, val):
+        Stack.push(self,val)
+        maxTop = self.getMaxTop()
+        if maxTop == None or maxTop <= val:
+            self.maxStack.push(val)
+        else:
+            self.maxStack.push(maxTop)
+    
+    """
+    pop()
+        1) Pop an element from maxStack
+        2) Pop and return an element from Special Stack
+    """
+    def pop(self):
+        self.maxStack.pop()
+        return Stack.pop(self)
+
+    def getMaxTop(self):
+        return self.maxStack.peek()
+
+    def max(self):
+        return self.getMaxTop()
+
+
+def main():
+    s = SpecialStack()
+    
+    s.push(18)
+    print(s, s.maxStack, s.max())
+    s.push(19)
+    print(s, s.maxStack, s.max())
+    s.push(29)
+    print(s, s.maxStack, s.max())
+    s.push(15)
+    print(s, s.maxStack, s.max())
+    s.push(16)
+    print(s, s.maxStack, s.max())
+
+    s.pop()
+    print(s, s.maxStack, s.max())
+    s.pop()
+    print(s, s.maxStack, s.max())
+    s.pop()
+    print(s, s.maxStack, s.max())
+    s.pop()
+    print(s, s.maxStack, s.max())
+    s.pop()
+    print(s, s.maxStack, s.max())
+
+if __name__ == "__main__":
+    main()

index.md

@@ -30,14 +30,13 @@
 * [#29 Run Length Encoding](./challenges/run-length-encoding.py)
 * [#36 Second Largest Node In BST](./challenges/second-largest-node-in-bst.py)
 * [#42 SubArray Sum](./challenges/subarray-sum.py)
+* [#43 Max Stack Implementation](./challenges/max-stack-implementation.py)
 * [#55 URL Shortener](./challenges/url-shortener.py)
 * [#157 Is Permutation Palindrome](./challenges/is-permutation-palindrome.py)
 * [#171 Busiest Time In Building](./challenges/busiest-time-in-building.py)
 * [#173 Flatten Nested Dictionary](./challenges/flatten-nested-dictionary.py)
 
 
-
-
 ## Lessons
 
 * [Merge Sort](./lessons/merge-sort.py)