练习作答

练习一: 给定一组连续的整数，例如：10，11，12，……，20，但其中缺失一个数字，试找出缺失的数字：

import java.util.*;

public class Main {
    public static void main(String[] args) {
        // 构造从start到end的序列：
        final int start = 10;
        final int end = 20;
        List<Integer> list = new ArrayList<>();
        for (int i = start; i <= end; i++) {
            list.add(i);
        }
        // 随机删除List中的一个元素:
        int removed = list.remove((int) (Math.random() * list.size()));
        int found = findMissingNumber(start, end, list);
        System.out.println(list.toString());
        System.out.println("missing number: " + found);
        System.out.println(removed == found ? "测试成功" : "测试失败");
    }
    static int findMissingNumber(int start, int end, List<Integer> list) {
        int found = 0;
        for (int i = 0; i < list.size(); i ++) {
            if (list.get(i) == start) {
                start ++;
                continue;
            }
            found = start;
        }
        return found;
    }
}

练习二: 增强版：和上述题目一样，但整数不再有序，试找出缺失的数字：

import java.util.*;

public class Main {
    public static void main(String[] args) {
        // 构造从start到end的序列：
        final int start = 10;
        final int end = 20;
        List<Integer> list = new ArrayList<>();
        for (int i = start; i <= end; i++) {
            list.add(i);
        }
        // 洗牌算法shuffle可以随机交换List中的元素位置:
        Collections.shuffle(list);
        // 随机删除List中的一个元素:
        int removed = list.remove((int) (Math.random() * list.size()));
        int found = findMissingNumber(start, end, list);
        System.out.println(list.toString());
        System.out.println("missing number: " + found);
        System.out.println(removed == found ? "测试成功" : "测试失败");
    }
    static int findMissingNumber(int start, int end, List<Integer> list) {
        Integer[] str = list.toArray(new Integer[list.size()]);
        Arrays.sort(str);
        int found = 0;
        for (int i = 0; i < str.length; i ++) {
            if (str[i] == start) {
                start ++;
                continue;
            }
            found = start;
        }
        return found;
    }
}