共有126种组合,用树状图来列,不容易出错!所有的组合如下：1234,1235,1236,1237,1238,1239,1245,1246,1247,1248,1249,1256,1257,1258,1259,1267,1268,1269,1278,1279,1289,1345,1346,1347,1348,1349,1356,1357,1358,1...