implement-stack-using-queues py3

0225_implement-stack-using-queues/README.md

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+Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (`push`, `top`, `pop`, and `empty`).
+
+Implement the `MyStack` class:
+
+*   `void push(int x)` Pushes element x to the top of the stack.
+*   `int pop()` Removes the element on the top of the stack and returns it.
+*   `int top()` Returns the element on the top of the stack.
+*   `boolean empty()` Returns `true` if the stack is empty, `false` otherwise.
+
+**Notes:**
+
+*   You must use **only** standard operations of a queue, which means that only `push to back`, `peek/pop from front`, `size` and `is empty` operations are valid.
+*   Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue's standard operations.
+
+**Example 1:**
+
+    Input
+    ["MyStack", "push", "push", "top", "pop", "empty"]
+    [[], [1], [2], [], [], []]
+    Output
+    [null, null, null, 2, 2, false]
+    
+    Explanation
+    MyStack myStack = new MyStack();
+    myStack.push(1);
+    myStack.push(2);
+    myStack.top(); // return 2
+    myStack.pop(); // return 2
+    myStack.empty(); // return False
+    
+
+**Constraints:**
+
+*   `1 <= x <= 9`
+*   At most `100` calls will be made to `push`, `pop`, `top`, and `empty`.
+*   All the calls to `pop` and `top` are valid.
+
+**Follow-up:** Can you implement the stack using only one queue?
+
+https://leetcode.com/problems/implement-stack-using-queues/

0225_implement-stack-using-queues/python3/two-queues-pop-linear.py

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+from collections import deque
+
+class MyStack:
+
+    def __init__(self):
+        self.q = deque()
+        self.tmp = deque()
+
+    def push(self, x: int) -> None:
+        self.q.append(x)
+        self.head = x
+
+    def pop(self) -> int:
+        tmp = deque()
+        popped = None
+        prev = None
+        
+        while True:
+            popped = self.q.popleft()
+            
+            if not self.q:
+                break
+            
+            prev = popped
+            tmp.append(prev)
+            
+        self.q = tmp
+        self.head = prev
+        
+        return popped
+        
+
+    def top(self) -> int:
+        return self.head
+
+    def empty(self) -> bool:
+        return len(self.q) == 0
+
+
+# Your MyStack object will be instantiated and called as such:
+# obj = MyStack()
+# obj.push(x)
+# param_2 = obj.pop()
+# param_3 = obj.top()
+# param_4 = obj.empty()