LeetCode 225. Implement Stack using Queues

225. Implement Stack using Queues

My Submissions QuestionEditorial Solution

Total Accepted: 37060 Total Submissions: 121389 Difficulty: Easy

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).

Update (2015-06-11):

The class name of the Java function had been updated to MyStack instead of Stack.

Credits:

Special thanks to @jianchao.li.fighter for adding this problem and all test cases.

class Stack {
public:
// Push element x onto stack.
queue<int> q1, q2;
void push(int x) {
q1.push(x);
}

// Removes the element on top of the stack.
void pop() {
while(q1.size() != 1) {
int t = q1.front();
q1.pop();
q2.push(t);
}
q1.pop();
while(!q2.empty()) {
int t = q2.front();
q2.pop();
q1.push(t);
}
}

// Get the top element.
int top() {
return q1.back();
}

// Return whether the stack is empty.
bool empty() {
return q1.empty();
}
};