181：36进制加法

class Solution {
    int to_int(char ch) {
        int x;
        if ('0' <= ch && ch <= '9') {
            x = ch - '0';
        } else {
            x = ch - 'a' + 10;
        }
        return x;
    }

    char to_char(int x) {
        char ch;
        if (0 <= x && x <= 9) {
            ch = x + '0';
        } else {
            ch = x - 10 + 'a';
        }
        return ch;
    }
public:
    string thirtysixAdd(string A, string B) {
        int m = A.size(), n = B.size();
        int i = m - 1, j = n - 1;
        int carry = 0;
        int x, y;
        string ans;
        while (i >= 0 || j >= 0 || carry > 0) {
            if (i >= 0) {
                x = to_int(A[i]);
            } else {
                x = 0;
            }
            if (j >= 0) {
                y = to_int(B[j]);
            } else {
                y = 0;
            }
            int tmp = x + y + carry;
            ans.push_back(to_char(tmp % 36));
            carry = tmp / 36;
            i--;
            j--;
        }
        reverse(ans.begin(), ans.end());
        return ans;
    }
};