大数相减

题目：

长度小于100 的两个数，默认第一个数比第二个数大，求差

代码如下：

package BigNumberSubtraction;

import java.util.Scanner;

public class BigNumberSubtraction {
	
	public String subMethod(String a,String b) {
		int aa[] = new int[a.length()];
		int bb[] = new int[b.length()];
		String result = "";
		for(int i=0;i<a.length();i++) {
			aa[i] = Integer.valueOf(String.valueOf(a.charAt(i)));
		} 
		for(int i=0;i<b.length();i++) {
			bb[i] = Integer.valueOf(String.valueOf(b.charAt(i)));
		}
		int index = b.length() - 1;
		for(int i=a.length()-1;i >= 0;i--) {
			if(index >= 0) {
				if(aa[i] >= bb[index])
					aa[i] = aa[i] - bb[index];
				else{
					int temp=1;
					while(aa[i-temp] == 0) {
						aa[i-temp] = 9;
						temp++;
					}
					aa[i-temp]--;
					aa[i] = aa[i] + 10 - bb[index];
				}
				index--;
			}
		}
		int start = 0;
		for(int i = 0;i<aa.length;i++) {
			if(aa[i]!=0) {
				start = i;
				break;
			}
		}
		for(int i=start;i<aa.length;i++) 
			result += aa[i];
		return result;
	}
	
	public static void main(String[] args) {
		//直接式
//		String a = "200000000000000000002"; //默认第一个数比第二个数大
//		String b = "100000000000000100001";
//		System.out.println(new BigNumberSubtraction().subMethod(a,b));
		//输入式
		String a = "";
		String b = "";
		Scanner cin = new Scanner(System.in);
		while(cin.hasNext()) {
			if(a.equals("")) {
				a = cin.nextLine();
			} else {
				b = cin.nextLine();
				System.out.println(new BigNumberSubtraction().subMethod(a,b));
				a = "";
				b = "";
			}
		}
	}
}

200000000000000000002 ?输入 100000000000000100001 ?输入 99999999999999900001 ? ?输出