442. Find All Duplicates in an Array

442. Find All Duplicates in an Array (Medium)

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?

Example:

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

Related Topics:
Array

Similar Questions:

• Find All Numbers Disappeared in an Array (Easy)

Solution 1.

// OJ: https://leetcode.com/problems/find-all-duplicates-in-an-array/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    vector<int> findDuplicates(vector<int>& A) {
        vector<int> ans;
        int i = 0, N = A.size();
        while (i < N) {
            if (A[i] == 0 || A[i] == i + 1) ++i;
            else if (A[i] == A[A[i] - 1]) {
                ans.push_back(A[i]);
                A[i++] = 0;
            } else swap(A[i], A[A[i] - 1]);
        }
        return ans;
    }
};