219. Contains Duplicate II 

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class Solution {
    public boolean containsNearbyDuplicate(int[] nums, int k) {
        // 窗口中元素数量：k + 1, eg: k = 1, window length = 2, when i = 1, start remove

        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 (i - k >= 0) {
                set.remove(nums[i - k]);
            }
        }

        return false;
    }
}

References

219. Contains Duplicate II