232.Implement-Queue-using-Stacks

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https://leetcode.com/problems/implement-queue-using-stacks/

https://www.lintcode.com/problem/implement-queue-by-two-stack

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``````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.

Example:
MyQueue queue = new MyQueue();
queue.push(1);
queue.push(2);
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

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).``````

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``````public class Solution {
private Stack<Integer> stack1;
private Stack<Integer> stack2;

public Solution() {
stack1 = new Stack<Integer>();
stack2 = new Stack<Integer>();
}

public void push(int element) {
stack1.push(element);
}

public int pop() {
if (stack2.empty()) {
stack1ToStack2(stack1, stack2);
}
return stack2.pop();
}

public int top() {
if (stack2.empty()) {
stack1ToStack2(stack1, stack2);
}
return stack2.peek();
}

private void stack1ToStack2() {
while (!stack1.empty()) {
stack2.push(stack1.pop());
}
}

}``````

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