[leetcode]: 415. Add Strings

1.题目

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.

两个字符串代表的数字，求加法的结果。不能使用内置的大整数操作或直接将字符串转为整数。

2.分析

从低位开始，按位来计算，跟手动算两个数的加法一样。注意进位，结果用字符串来存储结果。

3.代码

string addStrings(string num1, string num2) {    vector<char> sum;    int i = num1.size() - 1;    int j = num2.size() - 1;    int carry = 0;//进位    for (; i >= 0 || j >= 0; --i, --j) {        int x = i < 0 ? 0 : num1[i] - '0';        int y = j < 0 ? 0 : num2[j] - '0';        int tmp = x + y + carry;        sum.push_back(tmp % 10 + '0');        carry = tmp / 10;    }    if (carry > 0)        sum.push_back(carry + '0');    return string(sum.rbegin(), sum.rend());}