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| 1 | +#225. Implement Stack using Queues - LeetCode Solution Best Practice |
| 2 | +LeetCode link:[225. Implement Stack using Queues](https://leetcode.com/problems/implement-stack-using-queues), difficulty:**Easy** |
| 3 | + |
| 4 | +##Description of "225. Implement Stack using Queues" |
| 5 | +Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (`push`,`top`,`pop`, and`empty`). |
| 6 | + |
| 7 | +Implement the`MyStack` class: |
| 8 | + |
| 9 | +-`void push(int x)` Pushes element`x` to the top of the stack. |
| 10 | +-`int pop()` Removes the element on the top of the stack and returns it. |
| 11 | +-`int top()` Returns the element on the top of the stack. |
| 12 | +-`boolean empty()` Returns`true` if the stack is empty,`false` otherwise. |
| 13 | + |
| 14 | +**Notes**: |
| 15 | + |
| 16 | +- You must use**only** standard operations of a queue, which means that only`push to back`,`peek/pop from front`,`size` and`is empty` operations are valid. |
| 17 | +- Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue's standard operations. |
| 18 | + |
| 19 | + |
| 20 | +###[Example 1] |
| 21 | +**Input**: |
| 22 | +```javascript |
| 23 | +["MyStack","push","push","top","pop","empty"] |
| 24 | +[[], [1], [2], [], [], []] |
| 25 | +``` |
| 26 | + |
| 27 | +**Output**: |
| 28 | +```javascript |
| 29 | +[null,null,null,2,2,false] |
| 30 | +``` |
| 31 | + |
| 32 | +**Explanation**: |
| 33 | +```java |
| 34 | +MyStack myStack=newMyStack(); |
| 35 | +myStack.push(1); |
| 36 | +myStack.push(2); |
| 37 | +myStack.top();// return 2 |
| 38 | +myStack.pop();// return 2 |
| 39 | +myStack.empty();// return False |
| 40 | +``` |
| 41 | + |
| 42 | +###[Constraints] |
| 43 | +-`1 <= x <= 9` |
| 44 | +- At most`100` calls will be made to`push`,`pop`,`top`, and`empty`. |
| 45 | +- All the calls to`pop` and`top` are valid. |
| 46 | + |
| 47 | +###[Follow-up] |
| 48 | +Can you implement the stack using only one queue? |
| 49 | + |
| 50 | +##Intuition |
| 51 | +1. Two queues are used, one for input and output, and the other for temporary storage. |
| 52 | +2. There are two options for using queues to simulate the functions of a stack: |
| 53 | +1. Option 1: Simplify the`push(x)` operation and complicate the`pop()` and`top()` operations. When`pop()` or`top()`, you need to find the last data with effort. |
| 54 | +2. Option 2: Simplify the`pop()` and`top()` operations and complicate the`push(x)` operation. When`push(x)`, you need to insert`x` into the front of the queue with effort. |
| 55 | +3. Advantages of Option 2: Less code; easier to understand logically because it sorts the data in the queue according to the`last in, first out` rule. |
| 56 | +4. This article mainly introduces`Option 2`, and the code of`Option 1` is attached in`Python` to facilitate readers to compare the two solutions. |
| 57 | + |
| 58 | +##Complexity |
| 59 | +* Time:`push O(n)`,`pop O(1)`,`top O(1)`,`empty O(1)`. |
| 60 | +* Space:`O(n)`. |
| 61 | + |
| 62 | +##JavaScript |
| 63 | +```javascript |
| 64 | +varMyStack=function () { |
| 65 | +this.queue= [] |
| 66 | +this.queueTemp= [] |
| 67 | +}; |
| 68 | + |
| 69 | +MyStack.prototype.push=function (x) { |
| 70 | +while (this.queue.length>0) { |
| 71 | +this.queueTemp.push( |
| 72 | +this.queue.shift() |
| 73 | + ) |
| 74 | + } |
| 75 | + |
| 76 | +this.queue.push(x) |
| 77 | + |
| 78 | +while (this.queueTemp.length>0) { |
| 79 | +this.queue.push( |
| 80 | +this.queueTemp.shift() |
| 81 | + ) |
| 82 | + } |
| 83 | +}; |
| 84 | + |
| 85 | +MyStack.prototype.pop=function () { |
| 86 | +returnthis.queue.shift() |
| 87 | +}; |
| 88 | + |
| 89 | +MyStack.prototype.top=function () { |
| 90 | +returnthis.queue[0] |
| 91 | +}; |
| 92 | + |
| 93 | +MyStack.prototype.empty=function () { |
| 94 | +returnthis.queue.length===0 |
| 95 | +}; |
| 96 | +``` |
| 97 | + |
| 98 | +##Python |
| 99 | +###方案一：不推荐，仅用于对比。 |
| 100 | +```python |
| 101 | +classMyStack: |
| 102 | +def__init__(self): |
| 103 | +self.queue= deque() |
| 104 | +self.queue_temp= deque() |
| 105 | + |
| 106 | +defpush(self,x:int) ->None: |
| 107 | +self.queue.append(x) |
| 108 | + |
| 109 | +defpop(self) ->int: |
| 110 | +whilelen(self.queue)>1: |
| 111 | +self.queue_temp.append( |
| 112 | +self.queue.popleft() |
| 113 | + ) |
| 114 | + |
| 115 | + value=self.queue.popleft() |
| 116 | + |
| 117 | +whileself.queue_temp: |
| 118 | +self.queue.append( |
| 119 | +self.queue_temp.popleft() |
| 120 | + ) |
| 121 | + |
| 122 | +return value |
| 123 | + |
| 124 | +deftop(self) ->int: |
| 125 | + value=None |
| 126 | + |
| 127 | +whileself.queue: |
| 128 | + value=self.queue.popleft() |
| 129 | +self.queue_temp.append(value) |
| 130 | + |
| 131 | +whileself.queue_temp: |
| 132 | +self.queue.append( |
| 133 | +self.queue_temp.popleft() |
| 134 | + ) |
| 135 | + |
| 136 | +return value |
| 137 | + |
| 138 | +defempty(self) ->bool: |
| 139 | +returnlen(self.queue)==0 |
| 140 | +``` |
| 141 | + |
| 142 | +###方案二：推荐的方案。既短，又好理解。 |
| 143 | +```python |
| 144 | +classMyStack: |
| 145 | +def__init__(self): |
| 146 | +self.queue= deque() |
| 147 | +self.queue_temp= deque() |
| 148 | + |
| 149 | +defpush(self,x:int) ->None: |
| 150 | +whileself.queue: |
| 151 | +self.queue_temp.append( |
| 152 | +self.queue.popleft() |
| 153 | + ) |
| 154 | + |
| 155 | +self.queue.append(x) |
| 156 | + |
| 157 | +whileself.queue_temp: |
| 158 | +self.queue.append( |
| 159 | +self.queue_temp.popleft() |
| 160 | + ) |
| 161 | + |
| 162 | +defpop(self) ->int: |
| 163 | +returnself.queue.popleft() |
| 164 | + |
| 165 | +deftop(self) ->int: |
| 166 | +returnself.queue[0] |
| 167 | + |
| 168 | +defempty(self) ->bool: |
| 169 | +returnlen(self.queue)==0 |
| 170 | +``` |
| 171 | + |
| 172 | +##Java |
| 173 | +```java |
| 174 | +// Welcome to create a PR to complete the code of this language, thanks! |
| 175 | +``` |
| 176 | + |
| 177 | +##C++ |
| 178 | +```cpp |
| 179 | +// Welcome to create a PR to complete the code of this language, thanks! |
| 180 | +``` |
| 181 | + |
| 182 | +##C# |
| 183 | +```c# |
| 184 | +// Welcome to create a PR to complete the code of this language, thanks! |
| 185 | +``` |
| 186 | + |
| 187 | +##Go |
| 188 | +```go |
| 189 | +// Welcome to create a PR to complete the code of this language, thanks! |
| 190 | +``` |
| 191 | + |
| 192 | +##Ruby |
| 193 | +```ruby |
| 194 | +# Welcome to create a PR to complete the code of this language, thanks! |
| 195 | +``` |
| 196 | + |
| 197 | +##C, Kotlin, Swift, Rust or other languages |
| 198 | +``` |
| 199 | +// Welcome to create a PR to complete the code of this language, thanks! |
| 200 | +``` |