큐를 이용한 스택 구현

225. Implement Stack Using Queues

https://leetcode.com/problems/implement-stack-using-queues/

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.

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풀이. push()할 때, 큐를 이용해 재정렬

class MyStack:
    def __init__(self):
        self.q = collections.deque()

    def push(self, x):
        self.q.append(x)
        #요소 삽입 후 맨 앞에 두는 상태로 재정렬
        for _ in range(len(self.q) -1):
            self.q.append(self.q.popleft())

    def pop(self):
        return self.q.popleft()

    def top(self):
        return self.q[0]

    def empty(self):
        return len(self.q) == 0

유효한 괄호

21. Merge Two Sorted Lists

https://leetcode.com/problems/merge-two-sorted-lists/

Merge two sorted linked lists and return it as a sorted list. The list should be made by splicing together the nodes of the first two lists.

Example 1:

Input: l1 = [1,2,4], l2 = [1,3,4] Output: [1,1,2,3,4,4]

Example 2:

Input: l1 = [], l2 = [] Output: []

Example 3:

Input: l1 = [], l2 = [0] Output: [0]

Constraints:

• The number of nodes in both lists is in the range [0, 50].
• -100 <= Node.val <= 100
• Both l1 and l2 are sorted in non-decreasing order.

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풀이. 재귀 구조로 연결

def mergeTwoLists(self, l1:ListNode, l2: ListNode) -> ListNode:
    if (not l1) or (l2 and l1.val > l2.val):
        l1, l2 = l2, l1
    if l1:
        l1.next = self.mergeTwoLists(l1.next, l2)
    return l1