LeetCode Array Easy 448. Find All Numbers Disappeared in an Array

 Description

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]

 問題描述:給定一個數組，1 ≤ a[i] ≤ n (n爲數據的長度)  數組中有的元素出現一次，有的出現兩次，找到數組中沒有出現的元素

思路：先遍歷一遍作標記，而後再遍歷一次，找到沒有出現的元素

public IList<int> FindDisappearedNumbers(int[] nums) {
        //Array.Sort(nums);
        IList<int> res = new List<int>();
        for(int i = 0; i < nums.Length; i++){
            int index = Math.Abs(nums[i])-1;
            if(nums[index] > 0){
                nums[index] = -1 * nums[index];
            }
        }
        for(int i = 0; i < nums.Length; i++){
            if(nums[i] > 0){
                res.Add(i+1);
            }
        }
        return res;
    }