Find First Duplicate in an Array Java - The Coding Shala

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In this post, we will discuss how to find first duplicate in an array and will implement its solution in Java.

First Duplicate in an Array Problem

Given an array a that contains only numbers in the range from 1 to a.length, find the first duplicate number for which the second occurrence has the minimal index. In other words, if there are more than 1 duplicated numbers, return the number for which the second occurrence has a smaller index than the second occurrence of the other number does. If there are no such elements, return -1.

Example 1:
For a = [2, 1, 3, 5, 3, 2], the output should be firstDuplicate(a) = 3.
There are 2 duplicates: numbers 2 and 3. The second occurrence of 3 has a smaller index than the second occurrence of 2 does, so the answer is 3.

First Duplicate in an Array Java Solution

Approach 1:
We can use HashSet. If a number repeat, we will return that. It will take an additional O(n) space to store in HashSet.

Java Program: 
int firstDuplicate(int[] a) {
    HashSet<Integer> ch = new HashSet<>();
    for(int i=0;i<a.length;i++){
        int num = a[i];
        if(ch.contains(num)) return num;
        ch.add(num);
    }
    return -1;
}

Approach 2:
This approach is more efficient. we can modify the same array. In the given problem given numbers are in the range of 1 to a.length so we can modify the current number's sorted index. In this way, if a number repeats the number's sorted index is already modified.

Java Program: 
int firstDuplicate(int[] a) {
   for(int i=0;i<a.length;i++){
       if(a[Math.abs(a[i])-1]<0) return Math.abs(a[i]);
       a[Math.abs(a[i])-1] *= -1;
   }
   return -1;
}


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