观察字符串前后变化即可

class Solution {
public:
    string LeftRotateString(string str, int n) {
      if (str.empty() || n % str.size() == 0) {
        return str; 
      }
      
      n = n % str.size();
      
      std::reverse(str.begin(), str.begin() + n);
      std::reverse(str.begin() + n, str.end());
      std::reverse(str.begin(), str.end());
      
      return str;
    }
};