数组实现可以动态扩容的栈

用数组实现可以动态扩容的栈，详细代码如下：

package stack;

public class StackByArrayAutoCapacity {
    String[] arrays;
    int autoCapacity;
    int count;
    int index;

    public StackByArrayAutoCapacity(int n) {
        arrays = new String[n];
        autoCapacity = n;
        count = n;
        index = 0;
    }

    public boolean push(String item) {
        if (index == count) {
            String[] temp = arrays;
            arrays = new String[count + autoCapacity];
            for (int i =0;i<temp.length;i++) {
                arrays[i] = temp[i];
            }
            count = count + autoCapacity;
        }
        arrays[index] = item;
        index++;
        return true;
    }

    public String pop() {
        if (index == 0) {
            return null;
        }
        String temp = arrays[index - 1];
        index--;
        return temp;
    }

    public String peek() {
        if (index == 0) {
            return null;
        }
        String temp = arrays[index - 1];
        return temp;
    }

    public boolean empty() {
        return index == 0;
    }

}

复杂度分析

时间复杂度 : push 和 pop 均为：O(1)
空间复杂度 : push 和 pop 均为：O(1)