每一位上是3种选择,3*3*3*3=81种。
具体如下:
1111,1113,1119,1131,1133,1139,1191,1193,1199;
1311,1313,1319,1331,1333,1339,1391,1393,1399;
1911,1913,1919,1931,1933,1939,1991,1993,1999;
3111,3113,3119,3131,3133,3139,3191,3193,3199;
3311,3313,3319,3331,3333,3339,3391,3393,3399;
3911,3913,3919,3931,3933,3939,3991,3993,3999;
9111,9113,9119,9131,9133,9139,9191,9193,9199;
9311,9313,9319,9331,9333,9339,9391,9393,9399;
9911,9913,9919,9931,9933,9939,9991,9993,9999。
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