Python: Find the first duplicate element in a given array of integers

Last update on April 21 2025 12:52:12 (UTC/GMT +8 hours)

15. Find the First Duplicate Element in an Array

Write a Python program to find the first duplicate element in a given array of integers. Return -1 if there are no such elements.

Sample Solution:

Python Code :

def find_first_duplicate(nums):
    num_set = set()
    no_duplicate = -1

    for i in range(len(nums)):

        if nums[i] in num_set:
            return nums[i]
        else:
            num_set.add(nums[i])

    return no_duplicate

print(find_first_duplicate([1, 2, 3, 4, 4, 5]))
print(find_first_duplicate([1, 2, 3, 4]))
print(find_first_duplicate([1, 1, 2, 3, 3, 2, 2]))

Sample Output:

4
-1
1

For more Practice: Solve these Related Problems:

• Write a Python program to iterate through an array and return the first element that repeats, or -1 if none exist.
• Write a Python program to use a set to track seen elements and output the first duplicate encountered.
• Write a Python program to implement a function that returns both the first duplicate element and its index from an array.
• Write a Python program to use recursion to identify the first duplicate element in an array.

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• Python Array Exercises Home ↩
• Python Exercises Home ↩

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