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.

思路：用哈希表记录数字对应的位置，遍历数组，如果哈希表中已存在此数字，更新相距的最短距离，如果最短距离小于等于k，则返回true。

public boolean containsNearbyDuplicate(int[] nums, int k) {
    if (k < 1) {
        return false;
    }

    //record num => pos
    HashMap<Integer, Integer> map = new HashMap<>();

    //min distance
    int minDistance = Integer.MAX_VALUE;
    for (int i = 0; i < nums.length; i++) {
        if (map.containsKey(nums[i])) {
            minDistance = Math.min(minDistance, i - map.get(nums[i]));
            if (minDistance <= k) {
                return true;
            }
        }
        map.put(nums[i], i);
    }

    return false;
}