find-all-numbers-disappeared-in-an-array.cpp

// Time:  O(n)
// Space: O(1)

class Solution {
public:
    vector<int> findDisappearedNumbers(vector<int>& nums) {
        for (int i = 0; i < nums.size(); ++i) {
            if (nums[abs(nums[i]) - 1] > 0) {
                nums[abs(nums[i]) - 1] *= -1;
            }
        }

        vector<int> result;
        for (int i = 0; i < nums.size(); ++i) {
            if (nums[i] > 0) {
                result.emplace_back(i + 1);
            } else {
                nums[i] *= -1;
            }
        }
        return result;
    }
};