java-求100的阶乘-100!(即100*99*98*...*2*1),不用BigInteger - 代码物语-Let code talk - ITeye技术网站


java-求100的阶乘-100!(即100*99*98*...*2*1),不用BigInteger - 代码物语-Let code talk - ITeye技术网站
BigIntegerAddition.add见http://bylijinnan.iteye.com/blog/1463337
  1.     public static String factorial(int n){  
  2.         if(n<=0){  
  3.             return null;  
  4.         }  
  5.         String result="1";  
  6.         String tmp="0";  
  7.         for(int i=2;i<=n;i++){  
  8.             for(int j=0;j<i;j++){  
  9.                 //I write 'BigIntegerAddition' in my own BigIntegerAddition.java  
  10.                 tmp=BigIntegerAddition.add(result, tmp);//String add(String,String)  
  11.             }  
  12.             result=tmp;  
  13.             tmp="0";  
  14.         }  
  15.         return result;  
  16.     } 

http://bylijinnan.iteye.com/blog/1463337
  1.     //return x+y. Have not considered the negative number yet.  
  2.     public static String add(String x,String y){  
  3.         if(x==null||y==null){  
  4.             return null;  
  5.         }  
  6.         if(!isNumeric(x)||!isNumeric(y)){  
  7.             return null;  
  8.         }  
  9.         if(x.equals("0")){  
  10.             return y;  
  11.         }  
  12.         if(y.equals("0")){  
  13.             return x;  
  14.         }  
  15.           
  16.         if(x.length()>y.length()){  
  17.             String tmp=x;  
  18.             x=y;  
  19.             y=tmp;  
  20.         }  
  21.           
  22.         x=addZeroToFirst(x,y.length());  
  23.         String z=addHelp(x,y);  
  24.         return z;  
  25.     }  
  26.       
  27.     public static String addHelp(String x,String y){  
  28.         String z="";  
  29.         int len=x.length();  
  30.         int[] a=toIntArray(x);  
  31.         int[] b=toIntArray(y);  
  32.         int[] c=new int[len+1];  
  33.         int d=0;//to carry. No need to use int[]  
  34.         for(int i=0;i<len;i++){  
  35.             int tmpSum=a[len-1-i]+b[len-1-i]+d;  
  36.             c[len-i]=tmpSum%10;  
  37.             d=tmpSum/10;  
  38.         }  
  39.         c[0]=d;  
  40.         StringBuilder sb=new StringBuilder();  
  41.         for(int i=0;i<=len;i++){  
  42.             sb.append(c[i]);  
  43.         }  
  44.         if(c[0]==0){//delete the first '0' in result string  
  45.             z=sb.substring(1);  
  46.         }else{  
  47.             z=sb.toString();  
  48.         }  
  49.         return z;  
  50.     }  
  51.       
  52.   
  53.     //String - toCharArray - toIntArray  
  54.     public static int[] toIntArray(String str){  
  55.         int len=str.length();  
  56.         int[] result=new int[len];  
  57.         for(int i=0;i<len;i++){  
  58.             result[i]=str.charAt(i)-'0';  
  59.         }  
  60.         return result;  
  61.     }  
  62.       
  63.     //("123",5)-->"00123"  
  64.     public static String addZeroToFirst(String str,int length){  
  65.         StringBuilder sb=new StringBuilder();  
  66.         int diff=length-str.length();  
  67.         while(diff>0){  
  68.             sb.append("0");  
  69.             diff--;  
  70.         }  
  71.         sb.append(str);  
  72.         return sb.toString();  
  73.     }  
  74.       
  75.     public static boolean isNumeric(String str){  
  76.         Pattern p=Pattern.compile("[0-9]*");  
  77.         Matcher isNum=p.matcher(str);  
  78.         return isNum.matches();  
  79.     }  
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