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| 1 | +# 232. Implement Queue using Stacks |
| 2 | + |
| 3 | +- Difficulty: Easy. |
| 4 | +- Related Topics: Stack, Design, Queue. |
| 5 | +- Similar Questions: Implement Stack using Queues. |
| 6 | + |
| 7 | +## Problem |
| 8 | + |
| 9 | +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`). |
| 10 | + |
| 11 | +Implement the `MyQueue` class: |
| 12 | + |
| 13 | + |
| 14 | + |
| 15 | +- `void push(int x)` Pushes element x to the back of the queue. |
| 16 | + |
| 17 | +- `int pop()` Removes the element from the front of the queue and returns it. |
| 18 | + |
| 19 | +- `int peek()` Returns the element at the front of the queue. |
| 20 | + |
| 21 | +- `boolean empty()` Returns `true` if the queue is empty, `false` otherwise. |
| 22 | + |
| 23 | + |
| 24 | +**Notes:** |
| 25 | + |
| 26 | + |
| 27 | + |
| 28 | +- 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. |
| 29 | + |
| 30 | +- 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. |
| 31 | + |
| 32 | + |
| 33 | + |
| 34 | +Example 1: |
| 35 | + |
| 36 | +``` |
| 37 | +Input |
| 38 | +["MyQueue", "push", "push", "peek", "pop", "empty"] |
| 39 | +[[], [1], [2], [], [], []] |
| 40 | +Output |
| 41 | +[null, null, null, 1, 1, false] |
| 42 | + |
| 43 | +Explanation |
| 44 | +MyQueue myQueue = new MyQueue(); |
| 45 | +myQueue.push(1); // queue is: [1] |
| 46 | +myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue) |
| 47 | +myQueue.peek(); // return 1 |
| 48 | +myQueue.pop(); // return 1, queue is [2] |
| 49 | +myQueue.empty(); // return false |
| 50 | +``` |
| 51 | + |
| 52 | + |
| 53 | +**Constraints:** |
| 54 | + |
| 55 | + |
| 56 | + |
| 57 | +- `1 <= x <= 9` |
| 58 | + |
| 59 | +- At most `100` calls will be made to `push`, `pop`, `peek`, and `empty`. |
| 60 | + |
| 61 | +- All the calls to `pop` and `peek` are valid. |
| 62 | + |
| 63 | + |
| 64 | + |
| 65 | +**Follow-up:** Can you implement the queue such that each operation is **amortized** `O(1)` time complexity? In other words, performing `n` operations will take overall `O(n)` time even if one of those operations may take longer. |
| 66 | + |
| 67 | + |
| 68 | +## Solution |
| 69 | + |
| 70 | +```javascript |
| 71 | + |
| 72 | +var MyQueue = function() { |
| 73 | + this.stack1 = []; |
| 74 | + this.stack2 = []; |
| 75 | +}; |
| 76 | + |
| 77 | +/** |
| 78 | + * @param {number} x |
| 79 | + * @return {void} |
| 80 | + */ |
| 81 | +MyQueue.prototype.push = function(x) { |
| 82 | + this.stack1.push(x); |
| 83 | +}; |
| 84 | + |
| 85 | +/** |
| 86 | + * @return {number} |
| 87 | + */ |
| 88 | +MyQueue.prototype.pop = function() { |
| 89 | + if (this.stack2.length === 0) { |
| 90 | + while (this.stack1.length) this.stack2.push(this.stack1.pop()); |
| 91 | + } |
| 92 | + return this.stack2.pop(); |
| 93 | +}; |
| 94 | + |
| 95 | +/** |
| 96 | + * @return {number} |
| 97 | + */ |
| 98 | +MyQueue.prototype.peek = function() { |
| 99 | + if (this.stack2.length === 0) { |
| 100 | + while (this.stack1.length) this.stack2.push(this.stack1.pop()); |
| 101 | + } |
| 102 | + return this.stack2[this.stack2.length - 1]; |
| 103 | +}; |
| 104 | + |
| 105 | +/** |
| 106 | + * @return {boolean} |
| 107 | + */ |
| 108 | +MyQueue.prototype.empty = function() { |
| 109 | + return this.stack1.length === 0 && this.stack2.length === 0; |
| 110 | +}; |
| 111 | + |
| 112 | +/** |
| 113 | + * Your MyQueue object will be instantiated and called as such: |
| 114 | + * var obj = new MyQueue() |
| 115 | + * obj.push(x) |
| 116 | + * var param_2 = obj.pop() |
| 117 | + * var param_3 = obj.peek() |
| 118 | + * var param_4 = obj.empty() |
| 119 | + */ |
| 120 | +``` |
| 121 | + |
| 122 | +**Explain:** |
| 123 | + |
| 124 | +nope. |
| 125 | + |
| 126 | +**Complexity:** |
| 127 | + |
| 128 | +* Time complexity : O(n). |
| 129 | +* Space complexity : O(n). |
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