LintCode 40 Implement Queue By Two Stacks - Medium

40. Implement Queue By Two Stacks — Medium

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Problem

40. Implement Queue by Two Stacks

As the title described, you should only use two stacks to implement a queue's actions.

The queue should support push(element), pop() and top() where pop is pop the first(a.k.a front) element in the queue.

Both pop and top methods should return the value of first element.

Example Example 1:

Input: push(1) pop()
push(2) push(3) top()
pop()
Output: 1 2 2 Example 2:

Input: push(1) push(2) push(2) push(3) push(4) push(5) push(6) push(7) push(1) Output: [] Challenge implement it by two stacks, do not use any other data structure and push, pop and top should be O(1) by AVERAGE.

Notice Suppose the queue is not empty when the pop() function is called.

Solution

class MyQueue:
    
    def __init__(self):
        self.stack1 = []
        self.stack2 = []
    """
    @param: element: An integer
    @return: nothing
    """
    def push(self, element):
        self.stack1.append(element)
    """
    @return: An integer
    """
    def pop(self):
        if len(self.stack2) == 0:
            self.move()
        return self.stack2.pop()
    """
    @return: An integer
    """
    def top(self):
        if len(self.stack2) == 0:
            self.move()
        return self.stack2[-1]
    
    # 从1号栈转移到2号栈
    def move(self):
        while len(self.stack1) > 0:
            self.stack2.append(self.stack1.pop())