Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).
Implement the MyQueue class:
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void push(int x) Pushes element x to the back of the queue.
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int pop() Removes the element from the front of the queue and returns it.
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int peek() Returns the element at the front of the queue.
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boolean empty() Returns true if the queue is empty, false otherwise. Notes:
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You must use only standard operations of a stack, which means only push to top, peek/pop from top, size, and is empty operations are valid.
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Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack's standard operations.
Follow-up: Can you implement the queue such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.
Example 1:
Input
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 1, 1, false]
Explanation
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false
Constraints:
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1 <= x <= 9
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At most 100 calls will be made to push, pop, peek, and empty.
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All the calls to pop and peek are valid.
解法一：
使用两个栈。
class MyQueue() {
/** Initialize your data structure here. */
var stack1 = Stack<Int>()
var stack2 = Stack<Int>()
/** Push element x to the back of queue. */
fun push(x: Int) {
stack1.push(x)
}
/** Removes the element from in front of queue and returns that element. */
fun pop(): Int {
if (stack2.empty()) {
while (!stack1.empty()) {
val temp = stack1.pop()
stack2.push(temp)
}
}
return stack2.pop()
}
/** Get the front element. */
fun peek(): Int {
if (stack2.empty()) {
while (!stack1.empty()) {
val temp = stack1.pop()
stack2.push(temp)
}
}
return stack2.peek()
}
/** Returns whether the queue is empty. */
fun empty(): Boolean {
return stack1.empty() && stack2.empty()
}
}
/**
* Your MyQueue object will be instantiated and called as such:
* var obj = MyQueue()
* obj.push(x)
* var param_2 = obj.pop()
* var param_3 = obj.peek()
* var param_4 = obj.empty()
*/
