LeetCode_448. Find All Numbers Disappeared in an Array

448. Find All Numbers Disappeared in an Array

Easy

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]

package leetcode.easy;

public class FindAllNumbersDisappearedInAnArray {
	public java.util.List<Integer> findDisappearedNumbers(int[] nums) {
		boolean[] seenOrNot = new boolean[nums.length];
		for (int i = 0; i < seenOrNot.length; i++) {
			seenOrNot[i] = false;
		}
		for (int j = 0; j < nums.length; j++) {
			seenOrNot[nums[j] - 1] = true;
		}
		java.util.ArrayList<Integer> unseen = new java.util.ArrayList<Integer>();
		for (int i = 0; i < seenOrNot.length; i++) {
			if (!seenOrNot[i]) {
				unseen.add(i + 1);
			}
		}
		return unseen;
	}

	@org.junit.Test
	public void test() {
		int[] nums = { 4, 3, 2, 7, 8, 2, 3, 1 };
		System.out.println(findDisappearedNumbers(nums));
	}
}