Title
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
//cpp:
class MyQueue {
public:
/** Initialize your data structure here. */
stack<int> a, b;
MyQueue() {
}
/** Push element x to the back of queue. */
void push(int x) {
b.push(x);
}
/** Removes the element from in front of queue and returns that element. */
int pop() {
if (!a.empty()) {
int n = a.top();
a.pop();
return n;
}
else {
while (!b.empty()) {
a.push(b.top());
b.pop();
}
int n = a.top();
a.pop();
return n;
}
}
/** Get the front element. */
int peek() {
if (!a.empty()) {
int n = a.top();
return n;
}
else {
while (!b.empty()) {
a.push(b.top());
b.pop();
}
int n = a.top();
return n;
}
}
/** Returns whether the queue is empty. */
bool empty() {
return a.empty() && b.empty();
}
};