225. Implement Stack using Queues

Title

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

class Stack {
public:
// Push element x onto stack.
void push(int x) {
s1.push(x);
}

// Removes the element on top of the stack.
void pop() {
while(s1.size() > 1) {
s2.push(s1.front());
s1.pop();
}

if(s1.size() == 1) {
s1.pop();
}

while(!s2.empty()) {
s1.push(s2.front());
s2.pop();
}
}

// Get the top element.
int top() {
while(s1.size() > 1) {
s2.push(s1.front());
s1.pop();
}

if(s1.size() == 1) {
t = s1.front();
s2.push(s1.front());
s1.pop();
}

while(!s2.empty()) {
s1.push(s2.front());
s2.pop();
}

return t;
}

// Return whether the stack is empty.
bool empty() {
return s1.empty();
}

private:
queue <int> s1, s2;
int t;
};

run time: 0ms

两个队列