# LeetCode 217: Contains Duplicate — Step-by-Step Visual Trace

**Easy** — Array | Hash Table | Hash Set | Sorting

## The Problem

Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct.

## Approach

Use a hash set to track elements we've seen before. As we iterate through the array, check if the current element already exists in the set - if yes, we found a duplicate and return true, otherwise add it to the set.

**Time:** O(n) · **Space:** O(n)

## Code

```python
class Solution:
    def containsDuplicate(self, nums: List[int]) -> bool:
        hashset = set()

        for n in nums:
            if n in hashset:
                return True
            hashset.add(n)
        return False
```

## Watch It Run

> **[Open interactive visualization](https://tracelit.dev/app?trace=0217_contains-duplicate)**

> **Try it yourself:** Open [TraceLit](https://tracelit.dev/app?trace=0217_contains-duplicate) and step through every line.

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