贪心算法实例

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package recursion;

import java.util.HashMap;
import java.util.Map;

public class GreedyPackage {
    private static int  MAX_WEIGHT  = 150 ;
    private static int [] weights = new int[] {35,30,60,50,40,10,25};
    private static int [] values = new int[] {10,40,30,50,35,40,30};

    private void packageGreedy(int capacity , int weights[], int values[]) {
        int n = weights.length;
        double[] r = new  double[n];//性价比数组
        int index [] = new int[n];//性价比排序物品的下标
        for(int i =0;i<n;i++) {
            r[i] = (double)values[i]/(double)weights[i];
            index[i] = i;//默认排序                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
        }
        double  temp  =0;
        for(int i=0;i<n-1;i++) {
            for(int j =i+1;j<n;j++) {
                //性价比相同，取小的在前
                if(r[i]<r[j] || (r[i]==r[j] && weights[i]>weights[j])) {
                    temp = r[i];
                    r[i] = r[j];
                    r[j] = temp;

                    int x = index[i];
                    index[i] = index[j];
                    index[j] = x;
                }
            }
        }

        int []  w1 = new int[n];
        int []  v1 = new int[n];
        for(int i =0;i<n;i++) {
            w1[i] = weights[index[i]];
            v1[i] = values[index[i]];
        }

        int [] x = new int[n];//记录是否装进包了
        int  maxValue = 0;
        int  maxWeight = 0;
        int i =0;
        int  lastIndex = 0;
        while(i<n && w1[i]<capacity) {
            capacity -= w1[i]; 
            x[i] = 1;
            System.out.println("物品"+w1[i]+"被放进包了");
            maxValue += values[i];
            maxWeight += weights[i];
            lastIndex = i;
            i++;
        }
        //判断背包是否放满了，没有放满的就需要拿出最后的几个进行比较
        if(maxWeight<maxWeight+capacity && lastIndex<n-1) {
            //减去最后加的元素，并找到合适的元素替换
            Map<String,Integer> map = checkAndexchange(w1,v1,x,capacity,maxValue,maxWeight,lastIndex);
            if(null != map.get("MaxWeight")) {
                maxWeight = map.get("MaxWeight");
                maxValue = map.get("MaxValue");
            }
        }
        System.out.println("装进背包的最大价值为"+maxValue+"总重量为"+maxWeight);
    }
    /**
     * 这里仅仅优化了一步，多步优化就类似动态规划算法了
     * 
     * 用最后两个替换最后一个
     * 
     * 
     * w1 ,v1 是根据性价比排好的
     * @param w1  重量数组    
     * @param v1 价值数组
     * @param x 是否已经放进背包
     * @param capacity 背包剩余的容量
     * @param maxValue 背包里的物品的总价值
     *  @param maxWeight 背包里的物品的总重量
     * @param lastIndex 最后放进背包的元素的下标
     */
    private Map<String,Integer> checkAndexchange(int[] w1, int[] v1, int[] x, int capacity, int maxValue,int maxWeight , int lastIndex) {
        int value = v1[lastIndex];
        int weight = w1[lastIndex];
        int tempWeight = w1[lastIndex+1];
        int tempValue = v1[lastIndex+1];
        Map<String,Integer> map  = new HashMap<String,Integer>();
        for(int i=lastIndex+2;i<x.length;i++) {
            if(tempWeight+w1[i]<=weight+capacity && tempValue+v1[i]>value) {
                x[lastIndex] = 0;
                x[lastIndex+1] = 1;
                x[lastIndex+2] = 1;
                capacity = capacity+weight -tempWeight-w1[i];
                maxValue = maxValue+tempValue+v1[i]-value;
                maxWeight = maxWeight - weight+ tempWeight+w1[i];
                System.out.println("物品"+w1[lastIndex]+"被拿出包了");
                System.out.println("物品"+w1[lastIndex+1]+"被放进包了");
                System.out.println("物品"+w1[lastIndex+2]+"被放进包了");
                map.put("MaxValue", maxValue);
                map.put("MaxWeight", maxWeight);
            }
        }
        return map;
    }

    public static void main(String[] args) {
        GreedyPackage  gp = new GreedyPackage();
        gp.packageGreedy(MAX_WEIGHT, weights, values);
//      gp.packageGreedy(14, new int[] {5,5,7,6}, new int[] {10,10,13,11});
//      gp.packageGreedy(20, new int[] {16,10,10}, new int[] {32,19,18});
    }

}