题目:由6,7,8,9四个数字能组成多少个互不相同且无重复数字的三位数?分别是多少?
```
# 利用穷举法实现求解
NUM = 0;
numbers = [6,7,8,9]
for i in numbers:
for j in numbers:
for k in numbers:
if ((i!=j) and (i!=k) and (j!=k)):
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