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.

Notes:

You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is emptyoperations 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).

用栈来实现队列的功能

C++（3ms）：两个栈

 1 class MyQueue {
 2 public:
 3     stack<int> st1 ;
 4     stack<int> st2 ;
 5     /** Initialize your data structure here. */
 6     MyQueue() {
 7         
 8     }
 9     
10     /** Push element x to the back of queue. */
11     void push(int x) {
12         st1.push(x) ;
13     }
14     
15     /** Removes the element from in front of queue and returns that element. */
16     int pop() {
17         move();
18         int head = st2.top() ;
19         st2.pop() ;
20         return head ;
21     }
22     
23     /** Get the front element. */
24     int peek() {
25         move();
26         return st2.top() ;
27     }
28     
29     /** Returns whether the queue is empty. */
30     bool empty() {
31         return st1.empty() && st2.empty() ;
32     }
33     
34     void move(){
35         if (st2.size() <= 0){
36             while(st1.size() > 0){
37                 int t = st1.top() ;
38                 st1.pop() ;
39                 st2.push(t) ;
40             }
41         }
42     }
43 };
44 
45 /**
46  * Your MyQueue object will be instantiated and called as such:
47  * MyQueue obj = new MyQueue();
48  * obj.push(x);
49  * int param_2 = obj.pop();
50  * int param_3 = obj.peek();
51  * bool param_4 = obj.empty();
52  */

C++(0ms): 一个栈

 1 class MyQueue {
 2 public:
 3     stack<int> st; 
 4     /** Initialize your data structure here. */
 5     MyQueue() {
 6         
 7     }
 8     
 9     /** Push element x to the back of queue. */
10     void push(int x) {
11         pushHelper(x) ;
12     }
13     
14     void pushHelper(int x){
15         if (st.size() == 0){
16             st.push(x) ;
17             return ;
18         }
19         int t = st.top() ;
20         st.pop() ;
21         pushHelper(x) ;
22         st.push(t) ;
23         return ;
24     }
25     
26     /** Removes the element from in front of queue and returns that element. */
27     int pop() {
28         int t = st.top() ;
29         st.pop() ;
30         return t ;
31     }
32     
33     /** Get the front element. */
34     int peek() {
35         return st.top() ;
36     }
37     
38     /** Returns whether the queue is empty. */
39     bool empty() {
40         return st.empty() ;
41     }
42 };
43 
44 /**
45  * Your MyQueue object will be instantiated and called as such:
46  * MyQueue obj = new MyQueue();
47  * obj.push(x);
48  * int param_2 = obj.pop();
49  * int param_3 = obj.peek();
50  * bool param_4 = obj.empty();
51  */

来源：https://www.cnblogs.com/mengchunchen/p/8241279.html

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