Added Stack Code with examples

Author: imfaisalmalik

1_Stack/1_Stack.py

@@ -0,0 +1,38 @@
+class Stack:
+    def __init__(self):
+        self.top = -1
+        self.stack = []
+    
+    def push(self, data):
+        self.stack.append(data)
+        self.top += 1
+
+    def pop(self):
+        if self.top > -1:
+            data = self.stack[self.top]
+            del self.stack[self.top]
+            self.top -= 1
+            return data
+        else:
+            print("Stack is empty.")
+
+    def peek(self):
+        return self.stack[self.top]
+    
+    def __str__(self):
+        return str(self.stack)
+
+
+if __name__ == "__main__":
+    stack = Stack()
+    numbers = [5,15,25,35,45,55, 25, 65]
+
+    for num in numbers:
+        stack.push(num)
+    
+    print(f"Stack: {stack}")
+
+    print(f"Peek: {stack.peek()}")
+
+    for i in range(11):
+        print(f"Pop: {stack.pop()}")

1_Stack/1b_Stack.py

@@ -0,0 +1,53 @@
+'''
+PROBLEM: 
+
+Write a function in python that can reverse a string using stack data structure. 
+Use [Stack class]
+    ```
+    reverse_string("We will conquere COVID-19") should return "91-DIVOC ereuqnoc lliw eW"
+    ```
+'''
+
+class Stack:
+    def __init__(self):
+        self.top = -1
+        self.stack = []
+    
+    def push(self, data):
+        self.stack.append(data)
+        self.top += 1
+
+    def pop(self):
+        if self.top > -1:
+            data = self.stack[self.top]
+            del self.stack[self.top]
+            self.top -= 1
+            return data
+        else:
+            print("Stack is empty.")
+
+    
+    def reverse_string(self, txt):
+        print("pushing data.")
+        for i in txt:
+            self.push(i)
+        reverse_str = str()
+        print("pop data")
+        for i in range(self.top + 1):
+            reverse_str += self.pop()
+        
+        return reverse_str
+
+    
+    def __str__(self):
+        return str(self.stack)
+
+
+if __name__ == "__main__":
+    stack = Stack()
+    reverse_str = stack.reverse_string("We will conquere COVID-19")
+    print(reverse_str)
+    expected_result = "91-DIVOC ereuqnoc lliw eW"
+    print(reverse_str == expected_result)
+    print(stack)
+    

1_Stack/1c_Stack.py

@@ -0,0 +1,74 @@
+''' TODO : Write a function in python that checks if paranthesis in the string are balanced or not. 
+Possible parantheses are "{}',"()" or "[]". Use Stack.  
+    ```
+    is_balanced("({a+b})")     --> True
+    is_balanced("))((a+b}{")   --> False
+    is_balanced("((a+b))")     --> True
+    is_balanced("))")          --> False
+    is_balanced("[a+b]*(x+2y)*{gg+kk}") --> True
+    ```
+'''
+class Stack:
+    def __init__(self):
+        self.top = -1
+        self.stack = []
+    
+    def push(self, data):
+        self.stack.append(data)
+        self.top += 1
+
+    def pop(self):
+        if self.top > -1:
+            data = self.stack[self.top]
+            del self.stack[self.top]
+            self.top -= 1
+            return data
+        else:
+            print("Stack is empty.")
+    
+    def match_paranthesis(self, ch1, ch2):
+        paranthesis = {
+            "}": "{",
+            ")":"(", 
+            "]":"["
+        }
+        return paranthesis[ch1] == ch2
+
+    def is_balanced(self, txt):
+        print(txt, end=" >> ")
+        for ch in txt:
+            # print(ch, end=", ")
+
+            if ch in ["{" , "[" , "("]:
+                # print(f"detected character: {ch}")
+                self.push(ch)
+                # print(f"pushed: {self.stack}")
+            if ch in ["}" , "]" , ")"]:
+                if self.top == -1:
+                    return False 
+                elif not self.match_paranthesis(ch, self.pop()):
+                    return False
+            # print(f"self.top >> {self.top}")
+        return self.top == -1
+        
+    
+    def __str__(self):
+        return str(self.stack)
+
+
+if __name__ == "__main__":
+    stack = Stack()
+    print(stack.is_balanced("({a+b})"))
+    print(stack.is_balanced("))((a+b}{"))
+    print(stack.is_balanced("((a+b))"))
+    print(stack.is_balanced("((a+g))"))
+    print(stack.is_balanced("))(("))
+    print(stack.is_balanced("[a+b]*(x+2y)*{gg+kk}"))
+
+'''
+LOGIC >> Only push the starting paranthesis 
+Upon getting the ending paranthesis, pop the data and match.
+
+LIFO >> Paranthesis, e.g., ``(, {, [`` Last In >> First out ``], }, )``
+
+'''