P1157 Output of Combinations

Permutation and combination are common mathematical methods. A combination means selecting $r$ elements from $n$ elements (order does not matter and $r \le n$). We can simply think of the $n$ elements as the natural numbers $1, 2, \dots, n$, and choose any $r$ numbers from them. You are required to output all combinations. For example, when $n=5, r=3$, all combinations are: $123,124,125,134,135,145,234,235,245,345$.

A single line with two natural numbers $n, r$ ($1 < n < 21$, $0 \le r \le n$).

Output all combinations. Each combination occupies one line, and the elements in it are sorted in ascending order. Each element occupies a field width of three characters. All combinations are in lexicographical order. Note: When outputting, each number must have a field width of $3$. In C++, you can use the following code: ```cpp cout

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