[leetcode] 415. Add Strings 解題報告

題目鏈接：https://leetcode.com/problems/add-strings/

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

Note:

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

思路：簡單的高精度，可以將字符串翻轉之後從頭開始加，這樣看起來比較自然，當然也可以從尾部開始加．

代碼如下：

class Solution {
public:
    string addStrings(string num1, string num2) {
        reverse(num1.begin(), num1.end());
        reverse(num2.begin(), num2.end());
        string ans;
        int len1 = num1.size(), len2 = num2.size(), i = 0, flag = 0;
        while(i < len1 || i < len2)
        {
            int val = 0;
            if(i < len1) val += (num1[i] - '0');
            if(i < len2) val += (num2[i] - '0');
            ans += (val+flag)%10 + '0';
            flag = (val+flag)/10;
            i++;
        }
        if(flag) ans += '1';
        reverse(ans.begin(), ans.end());
        return ans;
    }
};