Implement Queue Using Stacks Problem
Description
LeetCode Problem 232.
Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).
Implement the MyQueue class:
- void push(int x) Pushes element x to the back of the queue.
- int pop() Removes the element from the front of the queue and returns it.
- int peek() Returns the element at the front of the queue.
- boolean empty() Returns true if the queue is empty, false otherwise.
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, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack’s standard operations.
Example 1:
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Input
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 1, 1, false]
Explanation
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false
Constraints:
- 1 <= x <= 9
- At most 100calls will be made to push, pop, peek, and empty.
- All the calls to pop and peek are valid.
Sample C++ Code
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class MyQueue {
public:
stack<int> s1, s2;
/** Initialize your data structure here. */
MyQueue() {
}
/** Push element x to the back of queue. */
void push(int x) {
s1.push(x);
return;
}
/** Removes the element from in front of queue and returns that element. */
int pop() {
int ele = 0;
while (!s1.empty()) {
s2.push(s1.top());
s1.pop();
}
if (!s2.empty()) {
ele = s2.top();
s2.pop();
}
while (!s2.empty()) {
s1.push(s2.top());
s2.pop();
}
return ele;
}
/** Get the front element. */
int peek() {
int ele = 0;
while (!s1.empty()) {
s2.push(s1.top());
s1.pop();
}
if (!s2.empty()) {
ele = s2.top();
}
while (!s2.empty()) {
s1.push(s2.top());
s2.pop();
}
return ele;
}
/** Returns whether the queue is empty. */
bool empty() {
return s1.empty();
}
};
/**
* Your MyQueue object will be instantiated and called as such:
* MyQueue* obj = new MyQueue();
* obj->push(x);
* int param_2 = obj->pop();
* int param_3 = obj->peek();
* bool param_4 = obj->empty();
*/