leetcode 232:Implement Queue using Stacks

Implement Queue using Stacks

Total Accepted: 93 Total Submissions: 256

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack -- which means onlypush to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

[思路]

用两个stacks实现一个queue.

[CODE]

class MyQueue {
    Stack<Integer> s1 = new Stack<>();
    Stack<Integer> s2 = new Stack<>();

    // Push element x to the back of queue.
    public void push(int x) {
        s1.push(x);
    }

    // Removes the element from in front of queue.
    public void pop() {
        if(!s2.isEmpty()) s2.pop();
        else {
            while(!s1.isEmpty()) s2.push(s1.pop());
            s2.pop();
        }
    }

    // Get the front element.
    public int peek() {
        if(!s2.isEmpty()) return s2.peek();
        else {
            while(!s1.isEmpty()) s2.push(s1.pop());
            return s2.peek();
        }
    }

    // Return whether the queue is empty.
    public boolean empty() {
        return s1.empty() && s2.empty();
    }
}