# SE103 Week 2 Post Session Solutions
[Implement Queue Using Stacks](#Implement-Queue-Using-Stacks)
TODO: Validate Stack Sequences
TODO: Sum of Subarray Minimums

## Implement Queue Using Stacks
**Java**

class MyQueue {

    Stack<Integer> input = new Stack();
    Stack<Integer> output = new Stack();

    public void push(int x) {
        input.push(x);
    }

    public void pop() {
        peek();
        output.pop();
    }

    public int peek() {
        if (output.empty())
            while (!input.empty())
                output.push(input.pop());
        return output.peek();
    }

    public boolean empty() {
        return input.empty() && output.empty();
    }
}

**Python**

class Queue(object):
    def __init__(self):
        self.inStack, self.outStack = [], []

    def push(self, x):
        self.inStack.append(x)

    def pop(self):
        self.move()
        self.outStack.pop()

    def peek(self):
        self.move()
        return self.outStack[-1]

    def empty(self):
        return (not self.inStack) and (not self.outStack)

    def move(self):
        if not self.outStack:
            while self.inStack:
                self.outStack.append(self.inStack.pop())