README.md

232. Implement Queue using Stacks

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

Note:

• 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).

Solutions (Rust)

1. Solution

struct MyQueue {
    stack_in: Vec<i32>,
    stack_out: Vec<i32>,
}


/**
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl MyQueue {

    /** Initialize your data structure here. */
    fn new() -> Self {
        Self {
            stack_in: Vec::new(),
            stack_out: Vec::new(),
        }
    }

    /** Push element x to the back of queue. */
    fn push(&mut self, x: i32) {
        self.stack_in.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    fn pop(&mut self) -> i32 {
        if self.stack_out.is_empty() {
            while !self.stack_in.is_empty() {
                self.stack_out.push(self.stack_in.pop().unwrap());
            }
        }
        self.stack_out.pop().unwrap()
    }

    /** Get the front element. */
    fn peek(&mut self) -> i32 {
        if self.stack_out.is_empty() {
            while !self.stack_in.is_empty() {
                self.stack_out.push(self.stack_in.pop().unwrap());
            }
        }
        *self.stack_out.last().unwrap()
    }

    /** Returns whether the queue is empty. */
    fn empty(&self) -> bool {
        self.stack_in.is_empty() && self.stack_out.is_empty()
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * let obj = MyQueue::new();
 * obj.push(x);
 * let ret_2: i32 = obj.pop();
 * let ret_3: i32 = obj.peek();
 * let ret_4: bool = obj.empty();
 */