Set mismatch in C++ (Assignment) incld

 class Solution {

public:

    vector<int> findErrorNums(vector<int>& nums) {

        int n = nums.size(); // size of array

        vector<int> count(n+1, 0); // to store frequency

        for(int i=0; i<n; i++) { // traverse array

            count[nums[i]]++; // increase count

        }

        int duplicate = 0, missing = 0;

        for(int i=1; i<=n; i++) { // from 1 to n

            if(count[i] == 2) duplicate = i;

            if(count[i] == 0) missing = i;

        }

        return {duplicate, missing};

    }

};

I faced difficulties while working on.. 1. finding the size of array:- nums.size(); is the correct syntax.

2. to store frequency of arrays:- vector<int> count(n+1, 0).

3. Traverse Array:- (int i=0; i<n; i++) // traverses array

                       count[nums[i]]++; // increases the count of traverse

4. in Leetcode or Coding interviews, always:- return {duplicate, missing};