LeetCode 641. Design Circular Deque

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:

class MyCircularDeque {
    int [] arr;
    int start;
    int end;
    int len;
    int k;
    
    /** Initialize your data structure here. Set the size of the deque to be k. */
    public MyCircularDeque(int k) {
        arr = new int[k];
        start = -1;
        end = -1;
        len = 0;
        this.k = k;
    }
    
    /** Adds an item at the front of Deque. Return true if the operation is successful. */
    public boolean insertFront(int value) {
        if(isFull()){
            return false;
        }
        
        if(start == -1){
            start = 0;
        }else{
            start = (start - 1 + k) % k;
        }
        
        arr[start] = value;
        if(end == -1){
            end = start;
        }
        
        len++;
        return true;
    }
    
    /** Adds an item at the rear of Deque. Return true if the operation is successful. */
    public boolean insertLast(int value) {
        if(isFull()){
            return false;
        }
        
        end = (end + 1) % k;
        arr[end] = value;
        if(start == -1){
            start = end;
        }
        
        len++;
        return true;
    }
    
    /** Deletes an item from the front of Deque. Return true if the operation is successful. */
    public boolean deleteFront() {
        if(isEmpty()){
            return false;
        }
        
        start = (start + 1) % k;
        len--;
        return true;
    }
    
    /** Deletes an item from the rear of Deque. Return true if the operation is successful. */
    public boolean deleteLast() {
        if(isEmpty()){
            return false;
        }
        
        end = (end - 1 + k) % k;
        len--;
        return true;
    }
    
    /** Get the front item from the deque. */
    public int getFront() {
        return isEmpty() ? -1 : arr[start];
    }
    
    /** Get the last item from the deque. */
    public int getRear() {
        return isEmpty() ? -1 : arr[end];
    }
    
    /** Checks whether the circular deque is empty or not. */
    public boolean isEmpty() {
        return len == 0;
    }
    
    /** Checks whether the circular deque is full or not. */
    public boolean isFull() {
        return len == k;
    }
}

/**
 * Your MyCircularDeque object will be instantiated and called as such:
 * MyCircularDeque obj = new MyCircularDeque(k);
 * boolean param_1 = obj.insertFront(value);
 * boolean param_2 = obj.insertLast(value);
 * boolean param_3 = obj.deleteFront();
 * boolean param_4 = obj.deleteLast();
 * int param_5 = obj.getFront();
 * int param_6 = obj.getRear();
 * boolean param_7 = obj.isEmpty();
 * boolean param_8 = obj.isFull();
 */

类似Design Circular Queue.