LeetCode-448. Find All Numbers Disappeared in an Array C#

Given an array of integers where 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.

Find all the elements of [1, n] inclusive that do not appear in this array.

Could you do it without extra space and in O(n) runtime? You may assume the returned list does not count as extra space.

Example:

Input:
[4,3,2,7,8,2,3,1]

Output:
[5,6]

Solution:

Mark the value of already apeared index to negative, then loop it through to find the positive ones , then the index of positive ones is the result;

 public class Solution {
     public IList<int> FindDisappearedNumbers(int[] nums) {
         int n = nums.Length;
         IList<int>  result = new List<int>();
         for(int i=; i<n;i++)
         {
             int index = Math.Abs(nums[i]);
             if(nums[index-]>)
             {
                 nums[index-]=-nums[index-];
             }
         }
         for(int i=; i<n;i++)
         {
             if(nums[i]>)
             {
                 result.Add(i+);
             }
         }
         return result;
     }
 }