Contains Duplicate II

Published: Oct 21, 2022

Easy Hash Table Sliding Window Array


This is not a difficult problem. However, making the solution efficient needs a bit of consideration. It can be solved by multiple O(n) iterations, but can be done just one O(n) loop. When we check all values from index 0 to n, it is ordered. What we care about is the last index of the same value only. When the same value appears, check the last index. If the difference is less than or equals to k, return True.

Problem Description

Given an integer array nums and an integer k, return true if there are two distinct indices i and j in the array such that nums[i] == nums[j] and abs(i - j) <= k.


  • 1 <= nums.length <= 10**5
  • -10**9 <= nums[i] <= 10**9
  • 0 <= k <= 10**5


Example 1
Input: nums = [1,2,3,1], k = 3
Output: true
Example 2
Input: nums = [1,0,1,1], k = 1
Output: true
Example 3
Input: nums = [1,2,3,1,2,3], k = 2
Output: false


The Hash Table is a good data structure to save the value and its last index pair. When the same value appears, it is in the Hash Table. So, check the difference between the last and current index. If it is less than or equals to k, the answer is True.


class Solution:
    def containsNearbyDuplicate(self, nums: List[int], k: int) -> bool:
        counts = {}
        for idx, v in enumerate(nums):
            if v in counts and idx - counts[v] <= k:
                return True
            counts[v] = idx
        return False


  • Time: O(n)
  • Space: O(n)