442. Find All Duplicates in an Array

Problem Statement

Find All Duplicates in an Array - LeetCode 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.

Pattern: Pattern 1-n range array

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Solution

class Solution {  
     
   public void swap(int[] x, int a, int b) {  
      int t = x[a];  
      x[a] = x[b];  
      x[b] = t;  
   }  
     
   public List<Integer> findDuplicates(int[] nums) {  
      // code here  
      int n = nums.length;  
      List<Integer> ans = new ArrayList<>();  
      //cycle sort  
      for (int i = 0; i < n; i++)  
         while (nums[i] != i + 1) {  
            int swapLoc = nums[i] - 1;  
            // if no. at swap location is same  
            if (nums[i] == nums[swapLoc])  
               break;  
            // else swap with correct loc  
            swap(nums, i, swapLoc);  
         }  
        
      // iterate over sorted array  
      for (int i = 0; i < n; i++) {  
         if(nums[i]!= i+1) ans.add(nums[i]);  
      }  
      return ans;  
   }  
}

Notes

Apply cyclic sort/self freq approach Cyclic Sort: Sort array, skip dupplicates, iterate over array all out of place elements are duplicates

Self Freq Approach: you know how. Python O(n) time O(1) space - LeetCode Discuss