LeetCode 448. Find All Numbers Disappeared in an Array

class Solution {
public:
    vector<int> findDisappearedNumbers(vector<int>& nums) {
        int n = nums.size();
        for (int num : nums) {
            int idx = (num - 1) % n;
            nums[idx] += n;
        }
        vector<int> ans;
        for (int i = 0; i < n; ++i)
            if (nums[i] <= n)
                ans.emplace_back(i + 1);
        return ans;
    }
};