京东面试题: 幸运数字4和7

题目描述

4和7是两个幸运数字，我们定义，十进制表示中，每一位只有4和7两个数的正整数都是幸运数字。前几个幸运数字是：4,7,44,47,74,77,444,447…

现在输入一个数字k，输出第k个幸运数

样例输入
3
5
100
10000000
样例输出
74
744747
44774447447477474444447

 

代码

public class Test {
    private static List list = new ArrayList<>();
    private static List sumList = new ArrayList<>();

    public static void main(String[] args) throws Exception {
        int n = 10000000;
        int sum = 0;
        int bits = 0;
        for (int i = 1; i < 30; i++) {
            if (i == 1) {
                list.add(2);
            } else {
                list.add(list.get(i - 2) * 2);
            }
            sum += list.get(i - 1);
            sumList.add(sum);
            if (sum > n) {
                bits = i;
                break;
            }
        }

        StringBuffer sb = new StringBuffer();
        n = n - sumList.get(bits - 2);
        for (int i = bits; i > 1; i--) {
            if (n > list.get(i - 1) / 2) {
                n = n - list.get(i - 1) / 2;
                sb.append("7");
            } else {
                sb.append("4");
            }
        }
        if (n > 1) {
            sb.append("7");
        } else {
            sb.append("4");
        }
        System.out.println(sb.toString());
    }
}