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Implement Stack using Queues
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Implement Stack using Queues
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/*
LeetCode 225:
Implement the following operations of a stack using queues.
push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
empty() -- Return whether the stack is empty.
Notes:
You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).
Solution:
Language: C++
Tag: stack queue
*/
class MyStack {
private:
// 1. two queues
queue<int> que1;
queue<int> que2;
int top1;
public:
/** Initialize your data structure here. */
MyStack() {
}
/** Push element x onto stack. */
void push(int x) {
// 1. O(1) time
que1.push(x);
top1 = x;
}
/** Removes the element on top of the stack and returns that element. */
int pop() {
// 1. O(n) time
while (que1.size() > 1) {
top1 = que1.front();
que2.push(top1);
que1.pop();
}
int tmp = que1.front();
que1.pop();
swap(que1, que2); // ????
return tmp;
}
/** Get the top element. */
int top() {
// 1. O(1) time
return top1;
}
/** Returns whether the stack is empty. */
bool empty() {
return (que1.empty() && que2.empty());
}
};
/**
* Your MyStack object will be instantiated and called as such:
* MyStack obj = new MyStack();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.top();
* bool param_4 = obj.empty();
*/