442. Find All Duplicates in an Array

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442. Find All Duplicates in an Array

Description

Difficulty: Medium

Related Topics: Array, Hash Table

Given an integer array nums of length n where all the integers of nums are in the range [1, n] and each integer appears once or twice, return an array of all the integers that appears twice.

You must write an algorithm that runs in O(n) time and uses only constant extra space.

Example 1:

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

Example 2:

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

Example 3:

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Input: nums = [1]
Output: []

Constraints:

  • n == nums.length
  • 1 <= n <= 105
  • 1 <= nums[i] <= n
  • Each element in nums appears once or twice.

Hints/Notes

  • flip the array element(negative) to mark if one element has appeared previously
  • we can use this solution because each element appears either once or twice

Solution

Language: C++

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