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find all duplicates in an array

admin 11月 12, 2020 0

Description

Given an array of integers, 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.
Find all the elements that appear twice in this array.
Could you do it without extra space and in O(n) runtime?

leetcode : https://leetcode.com/problems/find-all-duplicates-in-an-array/

Example:

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

Output:
[2,3]

Solution

vector<int> findDuplicates(vector<int>& nums) {
	if (nums.empty()) return{};
	vector<int> res;
	int n = nums.size();
	for (int i = 0; i < n; i++)
		nums[i] -= 1;       
	int i = 0;
	while (i < n) {
		if (nums[i] != nums[nums[i]])
			swap(nums[i], nums[nums[i]]);   //swap elements to their respective indexes.
		else
			i++;
	}
	for (int i = 0; i < n; i++) {
		if (nums[i] != i)
			res.push_back(nums[i] + 1);     //get the elements which are at some other indexes (as they are occuring twice)
	}
	return res;
}