LEEDCODE 709转换成小写字母

class Solution {
public:
    string toLowerCase(string s) {
        int len = s.length();
        string a = "";
        for(int i = 0; i < len; i++)
        {
            if(s[i] >= 65 && s[i] <= 90)
            {
               a += (s[i] + 32);
            }
            else
                a += s[i];
        }
        // cout<
        return a;

    }
};