Largest Odd Number in String

<-E 1903> Largest Odd Number in String

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