225. Implement Stack using Queues

Easy
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?

解題

type MyStack struct {
queue1 []int
queue2 []int
}
​
​
func Constructor() MyStack {
return MyStack{}
}
​
​
func (this *MyStack) Push(x int) {
this.queue1 = append(this.queue1, x)
}
​
​
func (this *MyStack) Pop() int {
res := -1
​
for len(this.queue1) > 1 {
this.queue2 = append(this.queue2, this.queue1[0])
this.queue1 = this.queue1[1:]
}
​
res = this.queue1[0]
​
this.queue1 = this.queue2
this.queue2 = []int{}
​
return res
}
​
func (this *MyStack) Top() int {
res := -1
​
for len(this.queue1) > 0 {
if len(this.queue1) == 1 {
res = this.queue1[0]
}
this.queue2 = append(this.queue2, this.queue1[0])
this.queue1 = this.queue1[1:]
}
​
this.queue1 = this.queue2
this.queue2 = []int{}
​
return res
}
​
​
func (this *MyStack) Empty() bool {
return len(this.queue1) == 0 && len(this.queue2) == 0
}
​
​
/**
* Your MyStack object will be instantiated and called as such:
* obj := Constructor();
* obj.Push(x);
* param_2 := obj.Pop();
* param_3 := obj.Top();
* param_4 := obj.Empty();
*/