219. Contains Duplicate II - Easy

Given an array of integers and an integer k, find out whether there are two distinct indices i and j in the array such that nums[i] = nums[j] and the absolute difference between i and j is at most k.

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

improved hash table solution: keep a sliding window of k elements using Hash Table.

time = O(n), space = O( min(n, k) )

class Solution {
    public boolean containsNearbyDuplicate(int[] nums, int k) {
        Set<Integer> set = new HashSet<>();
        for(int i = 0; i < nums.length; i++) {
            if(set.contains(nums[i])) {
                return true;
            }
            set.add(nums[i]);
            if(set.size() > k) {
                set.remove(nums[i - k]);
            }
        }
        return false;
    }
}

naive hash table solution:

time = O(n), space = O(n)

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class Solution {
    public boolean containsNearbyDuplicate(int[] nums, int k) {
        Map<Integer, List<Integer>> map = new HashMap<>();
        for(int i = 0; i < nums.length; i++) {
            map.putIfAbsent(nums[i], new ArrayList<>());
            map.get(nums[i]).add(i);
        }
        
        for(Map.Entry<Integer, List<Integer>> entry : map.entrySet()) {
            List<Integer> idx = entry.getValue();
            for(int i = 0; i < idx.size() - 1; i++) {
                if(idx.get(i + 1) - idx.get(i) <= k) {
                    return true;
                }
            }
        }
        return false;
    }
}