cc150_3_3.java

import java.util.Stack;
import java.util.ArrayList;

public class SubStacks {
	private ArrayList<substack> stacks;
	private int sub_capacity;
	private int cur_stack;
	
	private class substack<T>{
		Stack<T> stack;
		int N;
		
		public substack(){
			this.stack = new Stack<T>();
			this.N = 0;
		}
		
		public void push(T item){
			stack.add(item);
			N ++;
			
		}
		public T pop(){
			if (N == 0) return null;
			N --;
			return stack.pop();
		}
		
	}
	
	public SubStacks(int threshold){
		stacks = new ArrayList<substack>();
		stacks.add(new substack());
		cur_stack = 0;
		this.sub_capacity = threshold;
		
	}
	
	public <T> void push(T item){
		substack temp = stacks.get(cur_stack);
		if(temp.N >= sub_capacity){
			substack new_temp = new substack();
			stacks.add(new_temp);
			temp = new_temp;
			cur_stack ++;
		}
		temp.push(item);
		
	}
	
	public <T> T pop(){
		while(stacks.get(cur_stack).N == 0 && cur_stack > 0) cur_stack --;
		return (T) stacks.get(cur_stack).pop();
	}
	
	public <T> T popAt(int index){
		if(index > cur_stack) return null;
		return (T) stacks.get(index).pop();
	}
	
	public static void main(String[] args){
		
		SubStacks test = new SubStacks(10);
		for (int i = 0; i < 50; i++)
			test.push(i);
		

		System.out.println(test.popAt(2));
		System.out.println(test.popAt(2));
		System.out.println(test.popAt(2));
		
		
		System.out.println(test.popAt(3));
		for(int i = 0;i <10; i++){
			System.out.println(test.pop());
		}
		System.out.println(test.pop());
		
		
		
	}

}