[DP] 堆盒子问题

package chapter9;

import java.util.ArrayList;
import java.util.HashMap;

public class P10_book_ {

    
    public ArrayList<Box> createStack(Box[] boxes){
        return createStackR(boxes, null, new HashMap<Box, ArrayList<Box>>());
    }
    
    public ArrayList<Box> createStackR(Box[] boxes, Box bottom, 
            HashMap<Box, ArrayList<Box>> stackMap){
        
        if(stackMap.containsKey(bottom))
            return stackMap.get(bottom);
        
        ArrayList<Box> bestStack = new ArrayList<Box>();
        int bestHeight = 0;
        
        for(Box b : boxes){
            if(b.canBeAbove(bottom)){
                ArrayList<Box> newStack = createStackR(boxes, b, stackMap);
                int newHeight = stackHeight(newStack);
                
                if(newHeight > bestHeight){
                    bestHeight = newHeight;
                    bestStack = newStack;
                }
            }
        }
        
        // make a copy of bestStack before modify it
        bestStack = (ArrayList<Box>)bestStack.clone();
        
        if(bottom != null)
            bestStack.add(bottom);
        
        stackMap.put(bottom, bestStack);
        return bestStack;
    }
    
    
    
    public int stackHeight(ArrayList<Box> stack){
        
        if(stack == null || stack.isEmpty())
            return 0;
        
        int totalHeight = 0;
        for(Box b : stack){
            totalHeight += b.height;
        }
        return totalHeight;
    }
    
}

class Box{
    
    int width;
    int height;
    int depth;
    
    public boolean canBeAbove(Box box){
        if(box == null)
            return true;
        
        if(width < box.width && height < box.height && depth < box.depth){
            return true;
        }
        
        return false;
    }
}