#1 Daily Problems -First Duplicate

It’s never too late to start something new, to do all those things that you’ve been longing to do. So I have started to solve daily one Problem from any of following these Code signal, Hacker Rank like that

Problem: First Duplicate

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

• 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.
• For a = [2, 2], the output should be firstDuplicate(a) = 2;
• For a = [2, 4, 3, 5, 1], the output should be firstDuplicate(a) = -1.

Input/Output

• [input] array.integer a
• Guaranteed constraints:
  1 ≤ a.length ≤ 105,
  1 ≤ a[i] ≤ a.length.
• [output] integer
• The element in a that occurs in the array more than once and has the minimal index for its second occurrence. If there are no such elements, return -1.

My solution:

int firstDuplicate(int[] a) {
HashMap<Integer,Integer> hm=new HashMap<>();
for (int i=0;i<a.length;i++)
{
if (hm.get(a[i])==null)
hm.put(a[i],i);
else
return a[i];
}
return -1;
}

As everyone knows, No solution is optimal solution. So please just see my solution and tell me the correction to improve the things ✌ . Thank you guys