题解 | #挑7#
挑7
https://www.nowcoder.com/practice/ba241b85371c409ea01ac0aa1a8d957b
1.数学法，先找出7的倍数，再找出不是7的倍数，但数字里有7的情况，包括个数为7，对10取余数；千、百位为7的情况，把它除10，减少一位再重复判断；
import java.util.*;
public class Main {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
int n = in.nextInt();
int res = 0;
for (int i = 7; i <= n; i++) {
// 处理7的倍数情况
int num = i % 7;
if (num == 0) {
res++;
continue;
}
// 处理个位数为7的情况
num = i % 10;
if (num == 7) {
res++;
continue;
}
// 处理首位或者中间位数为7的情况
num = i;
while (num > 0) {
num = num / 10;
if (num == 7 || num % 10 == 7) {
res++;
break;
}
}
}
System.out.println(res);
}
}
2.先找出7的倍数，用String去判断有没有含7；
import java.util.*;
public class Main {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
int n = in.nextInt();
int res = 0;
for (int i = 7; i <= n; i++) {
// 处理7的倍数情况
int num = i % 7;
if (num == 0) {
res++;
continue;
}
// 处理有明文7出现的情况
String str = String.valueOf(i);
if(str.contains("7")){
res++;
}
}
System.out.println(res);
}
}
