难道就我这样想的吗(hahaha)

时间复杂度是 O(nlogn)
，空间复杂度是 O(n)
。

class Solution {
public:
    vector<int> findNumsAppearOnce(vector<int>& nums) {
        sort(nums.begin(),nums.end());
        vector<int> temp(nums);
        sort(temp.begin(),temp.end());
        nums.erase(unique(nums.begin(),nums.end()),nums.end());
        vector<int>ans;
        set_difference(temp.begin(),temp.end(),nums.begin(),nums.end(),inserter(ans,ans.begin()));
        vector<int>res;
        set_difference(nums.begin(),nums.end(),ans.begin(),ans.end(),inserter(res,res.begin()));
        return res;
    }
};