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 only push 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).
Solution:
class Queue {
public:
// Push element x to the back of queue.
stack<int> s;
bool cp = true;
void reverse(stack<int>& ss){
stack<int> t;
while(!ss.empty()){
t.push(ss.top());
ss.pop();
}
ss = t;
}
void push(int x) {
if(cp) s.push(x);
else{
reverse(s);
s.push(x);
cp = true;
}
}
// Removes the element from in front of queue.
void pop(void) {
if(cp){
reverse(s);
s.pop();
cp = false;
}else{
s.pop();
}
}
// Get the front element.
int peek(void) {
if(cp){
cp = false;
reverse(s);
return s.top();
}else{
return s.top();
}
}
// Return whether the queue is empty.
bool empty(void) {
return s.empty();
}
};
