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stacks/README.md

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+## stacks and queues
+
+<br>
+
+### stacks
+
+<br>
+
+* last in, first out structures (LIFO)
+* useful in certain recursive algorithms, where you can push temp data as you recurse, and remove them from the (memory or data structure) stack as you backtrace.
+
+
+<br>
+
+----
+
+### queues 
+
+<br>
+
+* first in, first out structures (FIFO), i.e., items are removed at the same order they are added.
+* queues can be implemented with two arrays or a dynamic array (linked list), as long as items are added and removed from opposite sides.
+* if implemented with a dynamic array, a more efficient solution is to use a circular queue (ring buffer), i.e. a fixed-size array and two pointers to indicate the starting and ending positions. an advantage of circular queues is that we can use the spaces in front of the queue. in a normal queue, once the queue becomes full, we cannot insert the next element even if there is a space in front of the queue. but using the circular queue, we can use the space to store new values.
+* queues are often used in breath-first search (where you store a list of nodes to be processed) or when implementing a cache.
+
+
+<br>
+
+----
+
+### `Queues.py`
+
+<br>
+
+```python
+> python3 queues.py
+
+🧪 Testing Queue...
+Is the queue empty? True
+Adding 1 to 10 in the queue...
+Queue: [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
+
+Queue size: 10
+Queue peek : 1
+Is the queue empty? False
+
+Dequeue...
+Queue: [10, 9, 8, 7, 6, 5, 4, 3, 2]
+
+Queue size: 9
+Queue peek: 2
+Is the queue empty? False
+
+
+🧪 Testing Priority Queue...
+Priority Queue: [(-4, 1, Item 4), (-1, 0, Item 1), (-3, 2, Item 3)]
+Pop: Item 4
+Priority Queue: [(-3, 2, Item 3), (-1, 0, Item 1)]
+```
+
+<br>
+
+### `stack.py`
+
+<br>
+
+```python
+python3 stack.py
+
+Testing Stack...
+
+Stack: [12, 13, 14, 15, 16, 17, 18, 19, 20]
+Stack size: 9
+Stack peek: 20
+Stack is empty: False
+Stack min: 12
+
+Popping...
+20
+19
+18
+17
+16
+Stack: [12, 13, 14, 15]
+```

stacks/Stack.py

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+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+# author: bt3gl
+
+
+class Stack:
+
+    def __init__(self):
+        self.content = []
+        self.min_array = []
+        self.min = float('inf')
+
+    ######################
+    #   Private methods
+    ######################
+    def __repr__(self):
+        return f'{self.content}'
+
+    ######################
+    #  Properties
+    ######################
+    @property
+    def size(self):
+        return len(self.content)
+    
+    @property
+    def peek(self):
+        if self.content:
+            return self.content[-1]
+        else:
+            print('❌ Queue is empty, cannot peek.')
+    
+    @property
+    def is_empty(self):
+        return not bool(self.content)
+
+    ######################
+    #   Public methods
+    ######################
+    def push(self, value):
+        if value < self.min:
+            self.min = value
+
+        self.content.append(value)
+        self.min_array.append(self.min)
+    
+    def pop(self):
+        if self.content:
+            value = self.content.pop()
+            self.min_array.pop()
+            if self.min_array:
+                self.min = self.min_array[-1]
+            return value
+    
+    def find_min(self):
+        if self.min_array:
+            return self.min_array[-1]
+
+
+if __name__ == '__main__':
+
+    ######################
+    #   Simple Stack
+    ######################
+    print('Testing Stack...')
+    stack = Stack()
+    for i in range(12, 21):
+        stack.push(i)
+    
+    print(f'\nStack: {stack}')
+    print(f'Stack size: {stack.size}')
+    print(f'Stack peek: {stack.peek}')
+    print(f'Stack is empty: {stack.is_empty}')
+    print(f'Stack min: {stack.find_min()}')
+    
+    print('\nPopping...')
+    for i in range(5):
+        print(stack.pop())
+    print(f'Stack: {stack}')