在1~9中选择4个数进行组合,有哪几种?

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。