448. Find All Numbers Disappeared in an Array

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448. Find All Numbers Disappeared in an Array

Description

Difficulty: Easy

Related Topics: Array, Hash Table

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:

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

Example 2:

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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.

Hints/Notes

  • make element at index negative to mark the element has been traversed

Solution

Language: C++

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class Solution {
public:
    vector<int> findDisappearedNumbers(vector<int>& nums) {
        for (int num : nums) {
            if (nums[abs(num) - 1] > 0) {
                nums[abs(num) - 1] *= -1;
            }
        }
        vector<int> res;
        for (int i = 0; i < nums.size(); i++) {
            if (nums[i] > 0) {
                res.push_back(i + 1);
            }
        }
        return res;
    }
};