Interview question 01.06. string compression

Subject requirements
String compression . The number of times characters are used to repeat , Write a method , Realize basic string compression function . such as , character string aabcccccaaa Will turn into a2b1c5a3. if “ Compress ” The string after does not get shorter , Returns the original string . You can assume that the string contains only uppercase and lowercase letters （a to z）.

 Example 1:
  Input ："aabcccccaaa"
  Output ："a2b1c5a3"
 Example 2:
  Input ："abbccd"
  Output ："abbccd"
  explain ："abbccd" Compressed as "a1b2c2d1", Longer than the original string length .
 Tips ：
 The string length is [0, 50000] Within the scope of 

class Solution {
    
public:
    string compressString(string S) {
    
        int len = S.size();
        string s1;
        int count = 0;
        for(int i = 0; i < len; i++)// Traversal string 
        {
    
            count++;// Number of times used to record letters 
            if(S[i] != S[i+1])// If the current letter is different from the next letter , Then store the letter and the number of times of the letter in s1 in 
            {
    
                s1+=S[i];
                s1 += to_string(count);
                count = 0;   
            }
        }
        if(len > s1.size())
            return s1;
        else
            return S;
    }
};

Experience
to_string(2), Convert numbers to character form