#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd(0x27, 16,2);

int thisChar = 'a';

void setup() {
  // set up the LCD's number of columns and rows:
  lcd.begin(16, 2);
  // turn on the cursor:
  lcd.cursor();
}

void loop() {
  // reverse directions at 'm':
  if (thisChar == 'm') {
    // go right for the next letter
    lcd.rightToLeft();
  }
  // reverse again at 's':
  if (thisChar == 's') {
    // go left for the next letter
    lcd.leftToRight();
  }
  // reset at 'z':
  if (thisChar > 'z') {
    // go to (0,0):
    lcd.home();
    // start again at 0
    thisChar = 'a';
  }
  // print the character
  lcd.write(thisChar);
  // wait a second:
  delay(1000);
  // increment the letter:
  thisChar++;
}