组合问题

问题描述：找出从自然数1，2，…，n中任取r个数的所有组合。

采用回溯法找问题的解，将找到的组合以从小到大顺序存于a[0]，a[1]，…，a[r-1]中，组合的元素满足以下性质：

（1） a[i+1]>a，后一个数字比前一个大；

（2） a-i<=n-r+1。 -Combinatorial problems Problem Descr iption: Find out from the natural numbers 1,2, ..., n r the number of either taking all the combinations. Problem using backtracking to find the solution, will find a combination of small to large order to save on a [0], a [1], ..., a [r-1], the combination of elements to meet the following properties: (1) a [i+1]>　a, after a large number than the previous one　(2) ai < = n-r+1.