输入一串字符，以“？”结束。统计各字母出现的次数，并按字母出现的多少输出 （先输出次数多的，次数相同的按字母表顺序输出，不出现的字母不输出）-Enter the string of characters to "?" End. Statistics of the number of letters appeared, and the number of letters appears output (first output frequency and number of the same