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.
- You must use only standard operations of a stack -- which means only
push to top
,peek/pop from top
,size
, andis 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 {
private:
stack<int> st, tmp;
public:
// Push element x to the back of queue.
void push(int x) {
st.push(x);
}
// Removes the element from in front of queue.
void pop(void) {
if (tmp.empty()) {
while (!st.empty()) {
tmp.push(st.top());
st.pop();
}
}
tmp.pop();
}
// Get the front element.
int peek(void) {
if (tmp.empty() && !st.empty()) {
while (!st.empty()) {
tmp.push(st.top());
st.pop();
}
}
return tmp.top();
}
// Return whether the queue is empty.
bool empty(void) {
if (st.empty() && tmp.empty())
return true;
return false;
}
};
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