/计算位数
计算阶乘
计算1!+2!+3!+···
计算1-1/3!+1/5!+···
计算pi pi/4=1-1/3+1/5+···
逆置字符串
将十进制转为十六进制/
public class pra{
public static int countNum(int n){
int count=0;
for(;n>0;n=n/10){
count++;
}
return count;
}
public static int factorial(int n){
if(n==1|n==0){
return 1;
}else{
return nfactorial(n-1);
}
}
public static int addFactorial(int n){
int sum=0;
for(int i=1;i<=n;i++){
sum=sum+factorial(i);
}
return sum;
}
public static double plusOrMinusAdd(int n){
double sum=0.0;
int j=1;
for(int i=1;i<=n;i=i+2){
sum=sum+j(1.0/factorial(i));
j=j-1;
}
return sum;
}
public static double calPi(int n){
double pi4=0.0;
for(int i=0;i<=n;i++){
int m=2i+1;
if(i%2==0){
pi4=pi4+1.0/m;
}else{
pi4=pi4-1.0/m;
}
}
return pi4*4;
}
public static String reverse(String s){
String r="";
for(int i=s.length()-1;i>=0;i--){
r=r+s.charAt(i);
}
return r;
}
public static String decToHex(int dec){
String hex="";
while(dec>=16){
int reminder=dec%16;
dec=dec/16;
if(reminder<10){
hex=hex+reminder;
}else{
char ch=(char)('A'+(reminder-10));
hex=hex+ch;
}
}
if(dec!=0){
hex=hex+dec;
}
return reverse(hex);
}
public static void main(String[] args){
int m=countNum(1234);
System.out.printf("%d%n",m);
int ret=factorial(5);
System.out.printf("%d%n",ret);
int a=addFactorial(3);
System.out.printf("%d%n",a);
double b=plusOrMinusAdd(5);
System.out.printf("%f%n",b);
double c=calPi(50);
System.out.printf("%f%n",c);
System.out.println(reverse("message"));
System.out.println(decToHex(21954));
}
}