class Solution {
public:
    string replaceSpaces(string &str) {
        if(str == " ")
            return str;
        int count = 0;
        for(auto& m : str)
            if(m == ' ') ++count;

        int n = str.size();
        str.resize(str.size() + 2*count);

        for(int i = n - 1; i >= 0; --i)
        {
            if(str[i] != ' ')
                str[i+count*2] = str[i];
            else
                {
                    str[i + count*2] = '0';
                    str[i + count*2 - 1] = '2';
                    str[i + count*2 - 2] = '%';
                    --count;
                }

        }

        return str;
    }
};