## Algorithm

Problem Name: 442. Find All Duplicates in an Array

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:

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

Example 2:

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

Example 3:

Input: nums = [1]
Output: []

Constraints:

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

## Code Examples

### #1 Code Example with Java Programming

Code - Java Programming

class Solution {
public List findDuplicates(int[] nums) {
List list = new ArrayList<>();
for (int i = 0; i < nums.length; i++) {
if (nums[Math.abs(nums[i]) - 1] < 0) {
}
nums[Math.abs(nums[i]) - 1] *= -1;
}
return list;
}
}
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Input

cmd
nums = [4,3,2,7,8,2,3,1]

Output

cmd
[2,3]

### #2 Code Example with Javascript Programming

Code - Javascript Programming

const findDuplicates = function(nums) {
if (nums === null || nums.length <= 1) {
return [];
}

let dup = [];
for (let i = 0, n = nums.length; i < n; i++) {
let next = Math.abs(nums[i]);
nums[next - 1] < 0 ? dup.push(next) : (nums[next - 1] = -nums[next - 1]);
}

return dup;
};

console.log(findDuplicates([4, 3, 2, 7, 8, 2, 3, 1]));
console.log(findDuplicates([10, 2, 5, 10, 9, 1, 1, 4, 3, 7]));
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Input

cmd
nums = [4,3,2,7,8,2,3,1]

Output

cmd
[2,3]

### #3 Code Example with Python Programming

Code - Python Programming

class Solution:
def findDuplicates(self, nums):
"""
:type nums: List[int]
:rtype: List[int]
"""
out=list()
for i in range(len(nums)):
if nums[abs(nums[i])-1]<0: out.append(abs(nums[i]))
else: nums[abs(nums[i])-1]*=-1
return out
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Input

cmd
nums = [1,1,2]

Output

cmd
[1]

### #4 Code Example with C# Programming

Code - C# Programming

using System;
using System.Collections.Generic;

namespace LeetCode
{
public class _0442_FindAllDuplicatesInAnArray
{
public IList FindDuplicates(int[] nums)
{
var result = new List();
for (int i = 0; i < nums.Length; i++)
{
var num = Math.Abs(nums[i]);
if (nums[num - 1] < 0)
else
nums[num - 1] *= -1;
}

return result;
}
}
}
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Input

cmd
nums = [1,1,2]

Output

cmd
[1]