1forjinrange(i+1,r):indices[j]=indices[j-1]+1yieldtuple(pool[i]foriinindices) 代码forcombinations()也可以表示为permutations
boollexicographical_compare(InputIterator1first1,InputIterator1last1,InputIterator2first2,InputIterator2last2,Comparecomp);//permutations