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easy
Blind 75NeetCode 150arrayshashing

Problem

Given a list of integers, decide whether any value appears more than once. Return true if at least one value is repeated, otherwise false.

Examples

Input: nums = [1, 2, 3, 1]
Output: true
The value 1 appears twice.
Input: nums = [1, 2, 3, 4]
Output: false
All values are unique.

Constraints

  • • 1 <= nums.length <= 10^5
  • • -10^9 <= nums[i] <= 10^9
Hints(tap to reveal)
  • 1. A set tracks what you've already seen.
  • 2. You can also sort and compare neighbours, trading time for space.
Optimal approach(spoiler)
  1. Iterate over the array, inserting each value into a hash set.
  2. Before inserting, check whether the value is already present.
  3. If it is, return true immediately.
  4. If the loop finishes, every value was unique, so return false.
  5. O(n) time, O(n) space.

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