[leetcode]: 415. Add Strings

1.題目

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.

兩個字元串代表的數字，求加法的結果。不能使用内置的大整數操作或直接将字元串轉為整數。

2.分析

從低位開始，按位來計算，跟手動算兩個數的加法一樣。注意進位，結果用字元串來存儲結果。

3.代碼

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