输出n个自然数中以r个数全排列,回溯法也称为试探法，该方法首先暂时放弃关于问题规模大小的限制，并将问题的候选解按某种顺序逐一枚举和检验。在回溯法中，放弃当前候选解，寻找下一个候选解的过程称为回溯。-The output n natural numbers r the number of full array backtracking also called heuristics, this method first temporarily abandon the restrictions on the size of the problem, and the problem of candidate solutions enumerate and test one by one in some order. Backtracking, abandoning the current candidate solutions, looking for the next candidate solution process is called backtracking.