Find All Numbers Disappeared in an Array

Posted 2020-08-16 amazingzoe

Given an array of integers, 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?

Example:

Input:

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

Output:

[5,6]

 

 1 class Solution {
 2 public:
 3     vector<int> findDisappearedNumbers(vector<int>& nums) {
 4         // put all numbers in correct positions 
 5         for (int i = 0; i < nums.size(); i++) {
 6             while (nums[i] != -1 && i != nums[i] - 1) {
 7                 if (nums[i] == nums[nums[i] - 1]) {
 8                     nums[i] = -1;
 9                     break;
10                 }
11                 swap(nums[i], nums[nums[i] - 1]);
12             }
13         }
14         
15         // put all vacant numbers into the result array
16         vector<int> result;
17         for (int i = 0; i < nums.size(); i++) {
18             if (nums[i] == -1)
19                 result.push_back(i + 1);
20         }
21         return result;
22     }
23 };