Find All Numbers Disappeared in an Array

Problem

Given an array nums of n integers where nums[i] is in the range [1, n], return an array of all the integers in the range [1, n] that do not appear in nums.

Example 1:

Input: nums = [4,3,2,7,8,2,3,1]
Output: [5,6]

Example 2:

Input: nums = [1,1]
Output: [2]

Constraints:

  • n == nums.length
  • 1 <= n <= 105
  • 1 <= nums[i] <= n

Follow up: Could you do it without extra space and in O(n) runtime? You may assume the returned list does not count as extra space.

Solution

class Solution {
    public List<Integer> findDisappearedNumbers(int[] nums) {
        // cyclic sort
        var i = 0;
        while (i < nums.length) {
            var dst = nums[i] - 1;
            if (nums[dst] == nums[i]) {
                i += 1;
            } else {
                swap(nums, i, dst);
            }
        }

        var ans = new ArrayList<Integer>();
        for (i = 0; i < nums.length; i++) {
            if (nums[i] != i + 1) {
                ans.add(i + 1);
            }
        }
        return ans;
    }

    void swap(int[] nums, int x, int y) {
        nums[x] ^= nums[y];
        nums[y] ^= nums[x];
        nums[x] ^= nums[y];
    }
}

Recent posts from blogs that I like

An Introduction to Google’s Approach to AI Agent Security

via Simon Willison

Notes on Cramer's rule

Cramer's rule is a clever solution to the classical system of linear equations Ax=b: \[\begin{bmatrix} a_{11} & a_{12} & a_{13} \\ a_{21} & a_{22} & a_{23} \\ a_{31} & a_{32} & a_{33} \\ \end{bmatrix} \begin{bmatrix}x_1 \\ x_2 \\ x_3\end{bmatrix} = \begin{bmatrix}b_1 \\ b_2 \\ b_3\end{bmatrix}\] Usi...

via Eli Bendersky

Brandjes: Paintings as witnesses to fires 1640-1813

Dramatic paintings of towns and cities on fire, usually at night, were popular during the Dutch Golden Age, and known as brandjes. Examples to well into the 19th century.

via The Eclectic Light Company