219. Contains Duplicate II

如果采用二重遍历会超时，所以需要用一些数据结构优化：

class Solution {
    public boolean containsNearbyDuplicate(int[] nums, int k) {
        Set<Integer> set= new HashSet<Integer>();
        for(int i=0;i<nums.length;i++){
            if(i>k) set.remove(nums[i-k-1]);   //只保留k+i - i + 1个元素
            if(!set.add(nums[i]))    //在此范围内加入重复的元素add会返回false
                return true;
        }
        return false;
    }
}

• Set集合里多个对象之间没有明显的顺序
• Set集合不允许重复元素,HashSet集合判断两个元素相等的标准是两个对象通过equals方法比较相等，并且两个对象的hashCode（）方法返回值也相等。

ASet represents a generic "set of values". A TreeSet is a set where the elements are sorted (and thus ordered), a HashSet is a set where the elements are not sorted or ordered. A HashSet is typically a lot faster than a TreeSet.
A TreeSet is typically implemented as ared-black tree, whereas a HashSet uses Object.hashCode() to create an index in an array. Access time for a red-black tree is O(log(n)) whereas access time for a HashSet ranges from constant-time to the worst case (every item has the same hashCode) where you can have a linear search time O(n).