Largest Odd Number in String — C++

// Time:  O(n)
// Space: O(1)

class Solution {
public:
    string largestOddNumber(string num) {
        for (int i = size(num) - 1; i >= 0; --i) {
            if ((num[i] - '0') % 2) {
                return num.substr(0, i + 1);
            }
        }
        return "";
    }
};