原题链接在这里：https://leetcode.com/problems/design-circular-deque/
题目：
Design your implementation of the circular double-ended queue (deque).
Your implementation should support following operations:
MyCircularDeque(k): Constructor, set the size of the deque to be k.insertFront(): Adds an item at the front of Deque. Return true if the operation is successful.insertLast(): Adds an item at the rear of Deque. Return true if the operation is successful.deleteFront(): Deletes an item from the front of Deque. Return true if the operation is successful.deleteLast(): Deletes an item from the rear of Deque. Return true if the operation is successful.getFront(): Gets the front item from the Deque. If the deque is empty, return -1.getRear(): Gets the last item from Deque. If the deque is empty, return -1.isEmpty(): Checks whether Deque is empty or not.isFull(): Checks whether Deque is full or not.
Example:
MyCircularDeque circularDeque = new MycircularDeque(3); // set the size to be 3 circularDeque.insertLast(1); // return true circularDeque.insertLast(2); // return true circularDeque.insertFront(3); // return true circularDeque.insertFront(4); // return false, the queue is full circularDeque.getRear(); // return 2 circularDeque.isFull(); // return true circularDeque.deleteLast(); // return true circularDeque.insertFront(4); // return true circularDeque.getFront(); // return 4
Note:
- All values will be in the range of [0, 1000].
- The number of operations will be in the range of [1, 1000].
- Please do not use the built-in Deque library.
题解：
Use an array to maintain values.
start index pointing to queue head, initialized as -1.
end index pointing to queue tail, initialized as -1.
When insertFront, if start == -1, assign start as 0, else start = (start - 1 + k) % k. Assign new value to arr[start]. If end is -1, need to update end = start. This only happens at the beginning.
When insertLast, end = (end + 1) % k. Assign new value to arr[end]. If start is -1, need to update start = end. This only happends at the begining.
deleteFront, move start + 1.
deleteLast, move end - 1.
Time Complexity: MyCircularDeque, O(1). insertFront, O(1). insertLast, O(1). deleteFront, O(1). deleteLast, O(1). getFront, O(1). getRear, O(1). isEmpty, O(1). isEmpty, O(1).
Space: O(k).
AC Java:
1 class MyCircularDeque {
2 int [] arr;
3 int start;
4 int end;
5 int len;
6 int k;
7
8 /** Initialize your data structure here. Set the size of the deque to be k. */
9 public MyCircularDeque(int k) {
10 arr = new int[k];
11 start = -1;
12 end = -1;
13 len = 0;
14 this.k = k;
15 }
16
17 /** Adds an item at the front of Deque. Return true if the operation is successful. */
18 public boolean insertFront(int value) {
19 if(isFull()){
20 return false;
21 }
22
23 if(start == -1){
24 start = 0;
25 }else{
26 start = (start - 1 + k) % k;
27 }
28
29 arr[start] = value;
30 if(end == -1){
31 end = start;
32 }
33
34 len++;
35 return true;
36 }
37
38 /** Adds an item at the rear of Deque. Return true if the operation is successful. */
39 public boolean insertLast(int value) {
40 if(isFull()){
41 return false;
42 }
43
44 end = (end + 1) % k;
45 arr[end] = value;
46 if(start == -1){
47 start = end;
48 }
49
50 len++;
51 return true;
52 }
53
54 /** Deletes an item from the front of Deque. Return true if the operation is successful. */
55 public boolean deleteFront() {
56 if(isEmpty()){
57 return false;
58 }
59
60 start = (start + 1) % k;
61 len--;
62 return true;
63 }
64
65 /** Deletes an item from the rear of Deque. Return true if the operation is successful. */
66 public boolean deleteLast() {
67 if(isEmpty()){
68 return false;
69 }
70
71 end = (end - 1 + k) % k;
72 len--;
73 return true;
74 }
75
76 /** Get the front item from the deque. */
77 public int getFront() {
78 return isEmpty() ? -1 : arr[start];
79 }
80
81 /** Get the last item from the deque. */
82 public int getRear() {
83 return isEmpty() ? -1 : arr[end];
84 }
85
86 /** Checks whether the circular deque is empty or not. */
87 public boolean isEmpty() {
88 return len == 0;
89 }
90
91 /** Checks whether the circular deque is full or not. */
92 public boolean isFull() {
93 return len == k;
94 }
95 }
96
97 /**
98 * Your MyCircularDeque object will be instantiated and called as such:
99 * MyCircularDeque obj = new MyCircularDeque(k);
100 * boolean param_1 = obj.insertFront(value);
101 * boolean param_2 = obj.insertLast(value);
102 * boolean param_3 = obj.deleteFront();
103 * boolean param_4 = obj.deleteLast();
104 * int param_5 = obj.getFront();
105 * int param_6 = obj.getRear();
106 * boolean param_7 = obj.isEmpty();
107 * boolean param_8 = obj.isFull();
108 */
原文地址：https://www.cnblogs.com/Dylan-Java-NYC/p/12079440.html