Added stack (#15)

* update project structure

* add stack

Author: realDuYuanChao

data_structures/stack/stack.py

@@ -0,0 +1,103 @@
+from typing import Any
+
+
+class Stack:
+    def __init__(self, capacity: int = 10) -> None:
+        self.stack = []
+        self.capacity = capacity
+
+    def __len__(self):
+        """
+        >>> stack = Stack()
+        >>> for i in range(0, 10):
+        ...     stack.push(i)
+        >>> len(stack)
+        10
+        """
+        return len(self.stack)
+
+    def size(self) -> int:
+        """
+        >>> stack = Stack()
+        >>> for i in range(0, 10):
+        ...     assert len(stack) == i
+        ...     stack.push(i)
+        >>> stack.size()
+        10
+        """
+        return len(self)
+
+    def is_full(self) -> bool:
+        """
+        >>> stack = Stack()
+        >>> for i in range(0, 10):
+        ...     stack.push(i)
+        >>> stack.is_full()
+        True
+        """
+        return len(self) == self.capacity
+
+    def is_empty(self) -> bool:
+        """
+        >>> stack = Stack()
+        >>> stack.is_empty()
+        True
+        >>> stack.push(666)
+        >>> stack.push(999)
+        >>> stack.is_empty()
+        False
+        """
+        return len(self) == 0
+
+    def push(self, item: Any) -> None:
+        """
+        >>> stack = Stack()
+        >>> for i in range(0, 10):
+        ...     stack.push(i)
+        >>> stack.push(666)
+        Traceback (most recent call last):
+        ...
+        ValueError: stack is full
+        """
+        if self.is_full():
+            raise ValueError("stack is full")
+        self.stack.append(item)
+
+    def pop(self) -> Any:
+        """
+        >>> stack = Stack()
+        >>> stack.pop()
+        Traceback (most recent call last):
+        ...
+        ValueError: stack is empty
+        >>> for i in range(0, 10):
+        ...     stack.push(i)
+        >>> for i in range(9, -1, -1):
+        ...     assert stack.pop() == i
+        """
+        if self.is_empty():
+            raise ValueError("stack is empty")
+        return self.stack.pop()
+
+    def peek(self) -> Any:
+        """
+        >>> stack = Stack()
+        >>> stack.peek()
+        Traceback (most recent call last):
+        ...
+        ValueError: stack is empty
+        >>> stack.push('a')
+        >>> stack.push('b')
+        >>> stack.push('c')
+        >>> stack.peek()
+        'c'
+        """
+        if self.is_empty():
+            raise ValueError("stack is empty")
+        return self.stack[-1]
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()