学步园

由于大数的阶乘的结果比较大，长整型是不能存储的下的，为了能实现这个功能，使用按位进行相乘，将没位的结果分开存放，最后输出，不过运算的速度有点慢，算2000！得好长时间，呵呵。思路如下：

24x5= 
   24 
x   5 
------ 
   20 
  10 
------ 
  120

代码如下：

import java.util.List;
import java.util.ArrayList;

class BigFactorial{
	public static void main(String args[]){
		BigFactorial bigFactorial = new BigFactorial();
		if(args.length >= 1){
			List<Integer> resultList = bigFactorial.computeFactorial(Integer.parseInt(args[0]));
			System.out.print("" + Integer.parseInt(args[0]) + "! = ");
			for( int i = 0; i < resultList.size(); i++){
				if( i == 0 && resultList.get(0) == 0){
				}
				else{
					System.out.print(""  + resultList.get(i));
				}
			}
		}
	}
	public List<Integer> computeFactorial(int paramNumber){
		List<Integer> localResult = new ArrayList<Integer>();
		if(paramNumber == 4){
			localResult.add(4);
			localResult.add(0, 2);
			return localResult;
		}
		else{
			List<Integer> multiplier = convertNumber(paramNumber);
			List<Integer> multiplicand = computeFactorial(paramNumber - 1);
			for(int i = multiplier.size() - 1; i >= 0; i--){
				for(int j = multiplicand.size() - 1; j >= 0; j--){
					int temp = 0;
					int result = multiplier.get(i) * multiplicand.get(j);
					if(result > 10){
						temp = result%10;
						result = result/10;
					}
					else{
						if(result == 10){
							result = 1;
							temp = 0;
						}
						else{
							temp = result;
							result = 0;
						}
					}
					if(localResult.size() == 0){
						localResult.add(temp);
						if(result != 0){
							localResult.add(0, result);
						}
					}
					else{
						int multiplierLength = multiplier.size() - 1 - i ;
						int multiplicandLength = multiplicand.size() - 1 - j;
						int insertNumber;
						int numberPast;
						if(multiplierLength == 0 || multiplicandLength == 0){
							numberPast = Math.max(multiplierLength, multiplicandLength);
						}
						else{
							numberPast = (multiplierLength + multiplicandLength);
						}
						insertNumber = localResult.size() - numberPast - 1;
						if(insertNumber == 0){
							int localParam1 = localResult.get(insertNumber) + temp;
							if(localParam1 >= 10){
								result = result + 1;
								localParam1 = localParam1%10;
							}
							localResult.set(insertNumber, localParam1);
							if(result != 0){
								localResult.add(0, result);
							}
						}
						else if(insertNumber == -1){
							localResult.add(0, temp);
							localResult.add(0, result);
						}
						else if(insertNumber == -2){
								localResult.add(0, 0);
								localResult.add(0, 0);
								localResult.add(0, temp);
								localResult.add(0, result);
						} 
						else{
							int localParam2 = localResult.get(insertNumber) + temp;
							if(localParam2 >= 10){
								result++;
								localParam2 = localParam2%10;
							}
							localResult.set(insertNumber, localParam2);
							if(insertNumber == 0){
								localResult.add(0, result);
							}
							else{
								localResult.set(insertNumber - 1, (localResult.get(insertNumber - 1) + result));
							}
						}
						for( int r = localResult.size() - 1; r >= 0; r--){
							if(r == 0 && localResult.get(0) == 0){
								localResult.remove(0);
							}
							if(localResult.get(r) >= 10){
								int temp1 = localResult.get(r)%10;
								int result1 = localResult.get(r)/10;
								if(r == 0){
									localResult.set(0, temp1);
									localResult.add(0, 1);
								}
								else{
									localResult.set(r, temp1);
									localResult.set(r - 1, localResult.get(r - 1) + result1);
								}
							}
						}
					}
				}
			}
			return localResult;
		}
	}
	
	private List<Integer> convertNumber(int paramNumber){
		int remainder = paramNumber%10;
		int dValue = (paramNumber - remainder)/10;
		List<Integer> convertedNumber = new ArrayList<Integer>();
		convertedNumber.add(remainder);
		paramNumber = dValue;
		while(dValue != 0){
			if(dValue == 10){
				remainder = 0;
				dValue = 1;
			}
			else{
				remainder = paramNumber%10;
				convertedNumber.add(0, remainder);
				dValue = (paramNumber - remainder)/10;
				paramNumber = dValue;
			}
		}
		return convertedNumber;
	}
}