LRU（Least Recently Used）置换算法

利用一个特殊的站保存当前使用的各个页面的页面号，每当进程访问某页面时，便将该页面的页面号从占中移出，将它压入栈顶。因此，栈顶始终是最新被访问页面的编号，而栈底则是最近最久未使用页面的页面号。

package test1;

import java.util.Stack;

public class LeastRecentPage
{
  private static Stack<Integer> cache;

    // METHOD SIGNATURE BEGINS, THIS METHOD IS REQUIRED
    public static int lruCountMiss(int max_cache_size, int[] pages) {
        
        int missCount = 0;
        
        if (cache == null) {
            cache = new Stack<Integer>();
        }

        int length = pages.length;
        for (int j = 0;j < length; j++){
            if(cache.contains(pages[j])){
                cache.removeElement(pages[j]);
                cache.push(pages[j]);
            }else {
                if(cache.size() == max_cache_size){
                    cache.remove(0);
                    cache.push(pages[j]);
                    missCount++;
                }else if(cache.size() < max_cache_size){
                    cache.push(pages[j]);
                    missCount++;
                }
            }
        }
    
        return missCount;
    }
  // METHOD SIGNATURE ENDS
    
    public static void main(String[] args) {
        int count = lruCountMiss(2,new int[]{2,3,1,3,2,1,4,3,2});
        System.out.println(count);
    }
}

java 在util下提供了Stack类，其中包含主要的方法：（摘自java api）

1. push

public E push(E item)

　　Pushes an item onto the top of this stack. This has exactly the same effect as:

 addElement(item)

　　Parameters:

　　item - the item to be pushed onto this stack.

　　Returns:

　　the item argument.

　　See Also:

　　Vector.addElement(E)

2. pop

public E pop()

　　Removes the object at the top of this stack and returns that object as the value of this function.

　　Returns:

The object at the top of this stack (the last item of the Vector object).

　　Throws:

EmptyStackException - if this stack is empty.

3. peek

public E peek()

　　Looks at the object at the top of this stack without removing it from the stack.

　　Returns:

the object at the top of this stack (the last item of the Vector object).

　　Throws:

EmptyStackException - if this stack is empty.

4. empty

public boolean empty()

　　Tests if this stack is empty.

　　Returns:

true if and only if this stack contains no items; false otherwise.

 5. search

public int search(Object o)

　　Returns the 1-based position where an object is on this stack. If the object o occurs as an item in this stack, this method returns the distance from the top of the stack of the 　　　　occurrence nearest the top of the stack; the topmost item on the stack is considered to be at distance 1. The equals method is used to compare o to the items in this stack.

　　Parameters:

o - the desired object.

　　Returns:

the 1-based position from the top of the stack where the object is located; the return value -1 indicates that the object is not on the stack.