LeetCode-Add Strings

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Description：
Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.

Note:

• The length of both num1 and num2 is < 5100.
• Both num1 and num2 contains only digits 0-9.
• Both num1 and num2 does not contain any leading zero.
• You must not use any built-in BigInteger library or convert the inputs to integer directly.

题意：给定两个用字符串表示的非负整数，要求计算两个字符串整数的和，并以字符串的形式返回；并且，不能够使用内置的函数来直接转化；

解法：在这里，我们对字符串从低位到高位进行求和，并且记录其进位，直到遍历完两个字符串的所有位；

Java

class Solution {
    public String addStrings(String num1, String num2) {
        StringBuilder result = new StringBuilder();
        int index1 = num1.length() - 1;
        int index2 = num2.length() - 1;
        int carry = 0;
        while (index1 >= 0 || index2 >= 0) {
            int x1 = index1 >= 0 ? num1.charAt(index1) - '0' : 0;
            int x2 = index2 >= 0 ? num2.charAt(index2) - '0' : 0;
            int add = (x1 + x2 + carry) % 10;
            result.insert(0, "" + add);
            carry = (x1 + x2 + carry) / 10;
            index1--;
            index2--;
        }
        if (carry != 0) result.insert(0, "" + carry);
        return result.toString();
    }
}