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,
1356,1357,1358,1359,
1367,1368,1369,
1378,1379,
1389,
1456,1457,1458,1459,
1467,1468,1469,
1478,1479,
1489,
1567,1568,1569,
1578,1579,
1589,
1678,1679,
1689,
1789,
2345,2346,2347,2348,2349,
2356,2357,2358,2359,
2367,2368,2369,
2378,2379,
2389,
2456,2457,2458,2459,
2467,2468,2469,
2478,2479,
2489,
2567,2568,2569,
2578,2579,
2589,
2678,2679,
2689,
2789,
3456,3457,3458,3459,
3467,3468,3469,
3478,3479,
3489,
3567,3568,3569,
3578,3579,
3589,
3678,3679,
3689,
3789,
4567,4568,4569,
4578,4579,
4589,
4678,4679,
4689,
4789,
5678,5679,
5689,
5789,
6789.