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此题与链表相加有异曲同工之妙

class Solution {
       public String addStrings(String num1, String num2) {
           if(num1 == "0") {
               return num2;        }        if(num2 == "0") {
               return num1;        }        int i = num1.length() - 1;        int j = num2.length() - 1;        int carry = 0;        int sum = 0;        String str = "";        while(i >= 0 || j >= 0 || carry != 0) {
               int a = i < 0? 0 : num1.charAt(i) - '0';//将字符转化为整数            int b = j < 0? 0 : num2.charAt(j) - '0';            sum = a + b + carry;            carry = sum / 10;//获取进位            sum = sum % 10;//获取各位            str = sum + str;            i--;            j--;        }        return str;    }}

class Solution {
       public String addStrings(String num1, String num2) {
           int i = num1.length() - 1, j = num2.length() - 1, add = 0;        StringBuffer ans = new StringBuffer();        while (i >= 0 || j >= 0 || add != 0) {
               int x = i >= 0 ? num1.charAt(i) - '0' : 0;            int y = j >= 0 ? num2.charAt(j) - '0' : 0;            int result = x + y + add;            ans.append(result % 10);            add = result / 10;            i--;            j--;        }        // 计算完以后的答案需要翻转过来        ans.reverse();        return ans.toString();    }}作者：LeetCode-Solution链接：https://leetcode-cn.com/problems/add-strings/solution/zi-fu-chuan-xiang-jia-by-leetcode-solution/来源：力扣（LeetCode）著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。