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]

 问题描述:给定一个数组，1 ≤ a[i] ≤ n (n为数据的长度)  数组中有的元素出现一次，有的出现两次，找到数组中没有出现的元素

思路：先遍历一遍做标记，然后再遍历一次，找到没有出现的元素

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;
    }