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DynamicArrayStack.java
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82 lines (70 loc) · 1.73 KB
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package stack;
/**
* 支持动态扩容的栈
*
* @param <T>
*/
public class DynamicArrayStack<T> implements Stack<T> {
private T[] items;
private int top;
private int capacity;
public DynamicArrayStack(int capacity) {
this.capacity = capacity;
this.items = (T[]) new Object[capacity];
this.top = 0;
}
// T(N)=O(1): 思考一下，这里涉及到了数组的动态扩容，
// 那么时间复杂度为什么仍然是O(1)
@Override
public boolean push(T item) {
if (top == capacity) {
capacity = capacity * 2;
resize(capacity);
}
items[top] = item;
top++;
return false;
}
// T(N)=O(1)
@Override
public T pop() {
if (top == 0) {
return null;
}
T item = items[top -1];
top--;
return item;
}
// T(N)=O(1)
@Override
public T peek() {
if (top == 0) {
return null;
}
return items[top -1];
}
private void resize(int size) {
T[] tmp = (T[]) new Object[size];
for (int i = 0; i < items.length; i++) {
tmp[i] = this.items[i];
}
this.items = tmp;
}
public static void main(String[] args) {
DynamicArrayStack<String> stack = new DynamicArrayStack<>(4);
stack.push("A");
stack.push("B");
stack.push("C");
stack.push("D");
System.out.println(stack.peek());
stack.push("E");
System.out.println(stack.peek());
stack.pop();
System.out.println(stack.peek());
stack.pop();
stack.pop();
stack.pop();
stack.pop();
System.out.println(stack.peek());
}
}