两个大数相加(Java)* 1、是整数；* 2、两个数无限大，long都装不下；* 3、不能用BigInteger；* 4、不能用任何包装类提供的运算方法；* 5、两个数都是以字符串的方式提供。

 

 * 思路：* 字符串逐位相加，需要进位则进位处理，考虑两个问题：* 1、char怎么转换为integer, 减去'0'即可* 2、怎么处理对应位相加?反转字符串相加，正确处理进位即可，* 这样个位对应个位，十位对应十位，剩余的直接追加

public class TwoBigIntegerSum {public String twoBigIntegerSum(String str1, String str2) {if (str1 == null || "".equals(str1)) {return str2;}if (str2 == null || "".equals(str2)) {return str1;}StringBuilder sb = new StringBuilder();char[] arr1 = str1.toCharArray();char[] arr2 = str2.toCharArray();int index1 = str1.length() - 1;int index2 = str2.length() - 1;//缓存是否需要进位boolean carry = false;while (index1 >= 0 && index2 >= 0) {char cur1 = arr1[index1];char cur2 = arr2[index2];int sum = cur1 - '0' + cur2 - '0';sum = carry ? sum + 1 : sum;carry = sum >= 10 ? true : false;sb.append((char) ((sum % 10) + '0'));index1--;index2--;}//处理剩余元素while (carry || index1 >= 0 || index2 >= 0) {if (index1 >= 0) {int sum = arr1[index1] - '0' + (carry ? 1 : 0);carry = sum >= 10 ? true : false;sb.append((char) ((sum % 10) + '0'));index1--;}else if (index2 >= 0) {int sum = arr2[index2] - '0' + (carry ? 1 : 0);carry = sum >= 10 ? true : false;sb.append((char) ((sum % 10) + '0'));index2--;}else {sb.append('1');carry = false;}}return sb.reverse().toString();}public static void main(String[] args) {String str1 = "11111112345678911111";String str2 = "2222";System.out.println(new TwoBigIntegerSum().twoBigIntegerSum(str1,str2));System.out.println(new BigInteger(str1).add(new BigInteger(str2)));String str3 = "999928";String str4 = "72";System.out.println(new TwoBigIntegerSum().twoBigIntegerSum(str3, str4));System.out.println(new BigInteger(str3).add(new BigInteger(str4)));}
}