circular array

题目地址：

https://discuss.leetcode.com/topic/38428/design-a-circular-buffer/3

解题思路：

Three tricky points to note:

1. When overwriting a value, you must move the start pointer forward to read the oldest value during pop.
2. When ever the end reaches for either start or end pointer, you must set to 0 and check for case mentioned above.
3. Check for empty CB for pop operation.

代码：

public class CircularBuffer<T> {

    private T[] store;
    // Always designates the start of CB.    private int start;
    // Always designates the end of CB.    private int end;
    // 0 indicates empty, initialCapacity is full.    private int size;

    public CircularBuffer(int initialCapacity) {
        this.store = (T[]) new Object[initialCapacity];
    }

    public void push(T... vals) {
        for(int i=0;i<vals.length;i++)
            push(vals[i]);
    }

    // Inserts an item at the end.    public void push(T val) {
        if (end == store.length){
            end = 0;
        }
        store[end] = val;
        if (size < store.length){
            size++; // When CB has free space.        }
        else {
            // Means the CB is full.            start++; // Value of start was overwritten.            if (start == store.length){
                start = 0;
            }
        }
        end++;
    }

    // Removes an item from start.    public T pop() {
        if (size == 0){
            throw new RuntimeException("CircularBuffer is empty.");
        }
        size--;
        T val = store[start];
        store[start] = null;
        start++;
        if (start == store.length){
            start = 0;
        }
        return val;
    }

    // Reads the next item.    public T next() {
        if (size == 0){
            throw new RuntimeException("CircularBuffer is empty.");
        }
        return store[start];
    }

    // Returns true if not empty.    public boolean hasNext() {
        return size > 0;
    }

    public void print() {
        for (int n = 0; n < store.length - 1; n++){
            System.out.print(store[n] + "->");
        }
        System.out.println(store[store.length - 1]);
    }

    public static void main(String[] args) {
        CircularBuffer<Integer> cb = new CircularBuffer<>(5);
        cb.push(1,2,3,4,5);
        cb.print();
        cb.push(6);
        cb.print();
        for(int i=0;i<5;i++){
            System.out.println(cb.pop());
        }
// Completely empty.//        cb.print();// Is empty now, will throw exception.//        System.out.println(cb.pop());    }

}