docs: private String transform(String expression) {
          char[] arr = expression.toCharArray();
          for (int i = 0; i < arr.length; i++) {
          	char cc=arr[i];
              if (cc == '-') {
                  if (i == 0) {
                      arr[i] = '~';
                  } else {
                      char c = arr[i - 1];
                      if (c == '+' || c == '-' || c == '*' || c == '/' || c == '(' || c == 'E' || c == 'e') {
                          arr[i] = '~';
                      }
                  }
              }
          }
          if(arr[0]=='~'||arr[1]=='('){
              arr[0]='-';
              return "0"+new String(arr);
          }else{
              return new String(arr);
          }
      }