大数运算（不包括除法）

/**
 * 该算法实现了大数+、-、x，但是没有实现对负数的处理，因为处理负数可以通过判断大小去规避这个问题。另外除法没有实现
 * 
 * @author jie.xiang
 * 
 */
public class BigNumberCounter {
	public BigNumberCounter() {
	}
	/**
	 * 当大数位数不相等的时候，往前面添加0，使其位数一致
	 * 
	 * 对大数相减，不足即与前面借位。
	 * 
	 * @param beforNum
	 * @param afterNum
	 * @return
	 * @author jie.xiang
	 */
	public String subNumbers(String beforNum, String afterNum) {
		int bLen = beforNum.length();
		int aLen = afterNum.length();
		int maxLength = Math.max(bLen, aLen);
		int distance = Math.abs(bLen - aLen);
		StringBuffer result = new StringBuffer();
		if (bLen > aLen) {
			afterNum = getZeroNumbers(distance) + afterNum;
		} else {
			beforNum = getZeroNumbers(distance) + beforNum;
		}
		int carry = 0;
		for (int i = maxLength - 1; i >= 0; i--) {
			int a = Integer.parseInt(beforNum.charAt(i) + "") - carry;
			int b = Integer.parseInt(afterNum.charAt(i) + "");
			carry = 0;
			if (a < b) {
				a += 10;
				carry = 1;
				if (i == 0) {
					result.append("-");
				}
			}
			result.append(String.valueOf(a - b));
		}
		return result.reverse().toString();
	}
	/**
	 * 当大数位数不相等的时候，往前面添加0，使其位数一致
	 * 
	 * 相加的值，与10产生的余数为结果，除以十位进位的内容。
	 * 
	 * @param beforNum
	 * @param afterNum
	 * @return
	 * @author jie.xiang
	 */
	public String addNumbers(String beforNum, String afterNum) {
		int bLen = beforNum.length();
		int aLen = afterNum.length();
		int maxLength = Math.max(bLen, aLen);
		int distance = Math.abs(bLen - aLen);
		StringBuffer result = new StringBuffer();
		if (bLen > aLen) {
			afterNum = getZeroNumbers(distance) + afterNum;
		} else {
			beforNum = getZeroNumbers(distance) + beforNum;
		}
		int tempValue = 0;
		int carry = 0;
		for (int i = maxLength - 1; i >= 0; i--) {
			int a = Integer.parseInt(beforNum.charAt(i) + "");
			int b = Integer.parseInt(afterNum.charAt(i) + "");
			tempValue = a + b + carry;
			result.append(tempValue % 10);
			carry = tempValue / 10;
		}
		if (carry > 0) {
			result.append(carry);
		}
		return result.reverse().toString();
	}
	private String getCharNumbers(int length, String ch) {
		StringBuffer result = new StringBuffer();
		for (int i = 0; i < length; i++) {
			result.append(ch);
		}
		return result.toString();
	}
	private String getZeroNumbers(int length) {
		return getCharNumbers(length, "0");
	}
	/**
	 * 大数相乘，逐位相乘，每个值与10产生的余数为结果，除以十位进位的内容，每次的值与前一位相乘的值相乘
	 * 
	 * @param beforNum
	 * @param afterNum
	 * @return
	 * @author jie.xiang
	 */
	public String mutiNumbers(String beforNum, String afterNum) {
		int bLen = beforNum.length();
		int aLen = afterNum.length();
		int tempValue = 0;
		int carry = 0;
		String aNumber = "0";
		for (int i = aLen - 1; i >= 0; i--) {
			int a = Integer.parseInt(afterNum.charAt(i) + "");
			StringBuffer sb = new StringBuffer();
			String bNumber = "0";
			carry = 0;
			if (a == 0) {
				continue;
			}
			for (int j = bLen - 1; j >= 0; j--) {
				int b = Integer.parseInt(beforNum.charAt(j) + "");
				tempValue = a * b + carry;
				sb.append(tempValue % 10);
				carry = tempValue / 10;
			}
			if (carry > 0) {
				sb.append(carry);
			}
			bNumber = sb.reverse().toString() + getZeroNumbers(aLen - i - 1);
			bNumber = addNumbers(aNumber, bNumber);
			aNumber = bNumber;
		}
		return aNumber;
	}
	/**
	 * 过滤结果，去除多余的0
	 * 
	 * @param values
	 * @return
	 */
	private String getNoPrefixZero(String values) {
		String result = "";
		int i = 0;
		char zero = '0';
		for (; i < values.length(); i++) {
			if (values.charAt(i) != zero) {
				break;
			}
		}
		result += values.substring(i);
		if (result.length() == 0) {
			result = "0";
		}
		return result;
	}
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		BigNumberCounter counter = new BigNumberCounter();
		System.out.println(counter.addNumbers(
				"99999999999999999999999999999999999999999999", "1"));
		System.out.println(counter.mutiNumbers("9999999999999999999999999",
				"9999999999999999999999999999999"));
		String value = counter.subNumbers(
				"25411111111111111111111111111111111111111136",
				"343453534546546757676856");
		System.out.println(counter.getNoPrefixZero(value));
	}
}

这几天突然被算法题难住了，确实忘记了哦，好久没做了，现在有时间自己再写写这些基本问题，基础不能忘啊，呵呵