queue-with-two-stacks.js

/**
 * Complexity Analysis
 * push: O(1)
 * pop: Amortized O(1), Worst-case O(n)
 * empty: O(1)
 * peek: O(1)
 */
function Queue() {
  var stack1 = [];
  var stack2 = [];
  return {
    push(val) { // O(1) operation
      stack1.push(val);
    },
    pop(val) {
      if (stack2.length === 0) {
        while(stack1.length > 0) {
          stack2.push(stack1.pop());
        }
      }
      return stack2.pop();
    },
    // get this front element
    peek() {
      return stack2[stack2.length - 1] || stack1[0];
    },
    empty() {
      return stack1.length === 0 && stack2.length === 0;
    }
  };
}

const q = Queue();

q.push(3);
q.push(9);
q.push(12);

console.log(q.peek()); // 3

q.pop();

console.log(q.peek()); // 9