逆序对（只包含ABCD）

/** * Created by 叶 on 2017/4/4. */ public class ReverseOrder { public static void main ( String [] args ) { int aa = reverseNums ( "DBCBBA" ); System . out . println ( aa ); } public static int reverseNums ( String str ) { int totalResult = 0 ; int totalA = 0 ; int totalB = 0 ; int totalC = 0 ; int totalD = 0 ; int len = str . length (); int [] nums = new int [ len ]; for ( int i = 0 ; i < len ; i ++) { if ( str . charAt ( i ) == 'D' ) { totalD ++; nums [ i ] = 0 ; } else if ( str . charAt ( i ) == 'C' ) { totalC ++; nums [ i ] = totalD ; } else if ( str . charAt ( i ) == 'B' ) { totalB ++; nums [ i ] = totalD + totalC ; } else { totalA ++; nums [ i ] = totalD + totalC + totalB ; } } for ( int i = 0 ; i < len ; i ++) { totalResult += nums [ i ]; } return totalResult ; } }