解释贪心算法，并用Java代码展示

贪心算法是一种以贪心的思想为基础，按照一定的选择规则，每一步选择当前最优的解，并将其作为全局最优解的算法。其特点是具有简单性、高效性和局部最优性，但不一定能得到全局最优解。

以下是一个使用贪心算法求解背包问题的Java代码：

public class GreedyKnapsack {
    public static void main(String[] args) {
        int[] w = {5, 10, 20, 30}; // 物品重量
        int[] v = {50, 60, 140, 120}; // 物品价值
        int capacity = 50; // 背包容量
        double maxValue = solveGreedyKnapsack(w, v, capacity); // 解决背包问题
        System.out.println("最大价值为：" + maxValue);
    }

    public static double solveGreedyKnapsack(int[] w, int[] v, int capacity) {
        int n = w.length;
        // 计算物品单位价值
        double[] a = new double[n];
        for (int i = 0; i < n; i++) {
            a[i] = (double) v[i] / w[i];
        }
        // 按单位价值从大到小排序
        for (int i = 0; i < n - 1; i++) {
            for (int j = i + 1; j < n; j++) {
                if (a[j] > a[i]) {
                    double temp = a[i];
                    a[i] = a[j];
                    a[j] = temp;
                    temp = v[i];
                    v[i] = v[j];
                    v[j] = (int) temp;
                    temp = w[i];
                    w[i] = w[j];
                    w[j] = (int) temp;
                }
            }
        }
        // 装包
        double maxValue = 0;
        int weight = 0;
        for (int i = 0; i < n && weight < capacity; i++) {
            if (w[i] < capacity - weight) {
                maxValue += v[i];
                weight += w[i];
            } else {
                maxValue += a[i] * (capacity - weight);
                weight = capacity;
            }
        }
        return maxValue;
    }
}