Idea

模拟

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