Given a collection of numbers that might contain duplicates, return all possible unique permutations. For example,[1,1,2] have the following unique permutations: [ [1,1,2], [1,2,1], [2,1,1] ]和一般的Permutation不一样的是,这种permutation需要排序,使相同的元素能够相邻,选取下一个元素的时…