[LeetCode]448 Find All Numbers Disappeared in an Array

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]

Java
accepted simple solution

The basic idea is that we iterate through the input array and mark elements as negative using

nums[nums[i]
-1] = -nums[nums[i]-1]

. In this way all the numbers that we have seen will be marked as negative. In the second iteration, if a value is not marked as negative, it implies we have never seen that index before, so just add it to the return list.

public List<Integer> findDisappearedNumbers(int[] nums) {
List<Integer> ret = new ArrayList<Integer>();

for(int i = 0; i < nums.length; i++) {
int val = Math.abs(nums[i]) - 1;
if(nums[val] > 0) {
nums[val] = -nums[val];
}
}

for(int i = 0; i < nums.length; i++) {
if(nums[i] > 0) {
ret.add(i+1);
}
}
return ret;
}

Simple
Java In-place sort solution

The idea is simple, if nums[i] != i + 1 and nums[i] != nums[nums[i] - 1], then we swap nums[i] with nums[nums[i] - 1], for example, nums[0] = 4 and nums[3] = 7, then we swap nums[0] with nums[3]. So In the end the
array will be sorted and if nums[i] != i + 1, then i + 1 is missing.

The example run as follows

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

Since every swap we put at least one number to its correct position, the time is O(n)

public class Solution {
public List<Integer> findDisappearedNumbers(int[] nums) {
for (int i = 0; i < nums.length; i++) {
while (nums[i] != i + 1 && nums[i] != nums[nums[i] - 1]) {
int tmp = nums[i];
nums[i] = nums[tmp - 1];
nums[tmp - 1] = tmp;
}
}
List<Integer> res = new ArrayList<Integer>();
for (int i = 0; i < nums.length; i++) {
if (nums[i] != i + 1) {
res.add(i + 1);
}
}
return res;
}
}

5-line
Java Easy-understanding

public List<Integer> findDisappearedNumbers(int[] nums) {
List<Integer> res = new ArrayList<>();
int n = nums.length;
for (int i = 0; i < nums.length; i ++) nums[(nums[i]-1) % n] += n;
for (int i = 0; i < nums.length; i ++) if (nums[i] <= n) res.add(i+1);
return res;
}