Leetcode Find All Duplicates in an Array problem solution

YASH PAL, 31 July 202422 January 2026

In this Leetcode Find All Duplicates in an Array problem solution, we have 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.

Leetcode Find All Duplicates in an Array problem solution

Leetcode Find All Duplicates in an Array problem solution in Python.

class Solution(object):
    def findDuplicates(self, nums):
        result = []
        for x in nums:
            if nums[abs(x)-1] < 0:
                result.append(abs(x))
            else:
                nums[abs(x)-1] = -1*nums[abs(x)-1]
        return result

Find All Duplicates in an Array problem solution in Java.

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

Problem solution in C++.

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

Problem solution in C.

int* findDuplicates(int* nums, int numsSize, int* returnSize)

{   int arr[numsSize];

    for(int i = 0; i<numsSize; i++)  arr[i] = i+1;
 
    int c[numsSize];
    
   for(int i = 0; i<numsSize;i++)  c[i] = 0;
 
   int i = 0; 
   while(i<numsSize)
    {
         if (nums[i]==arr[nums[i]-1]) c[nums[i]-1]++ ;
         i++;
     }
 
 
int*a = (int*) malloc(numsSize*sizeof(int));
  
  int j = 0;
  for(int i = 0; i<numsSize; i++)
    {
        if(c[i]==2)    { a[j] =i+1;  j++; }        
    }
    
    a =  (int*) realloc(a,j*sizeof(int));
    *returnSize = j;
    return a;    
}

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