Given an array of integers and an integer k, find out whether there there are two distinct indices i and j in the array such that nums[i] = nums[j] and the difference between i and j is at most k.
解题思路：
HashMap，JAVA实现如下：
public boolean containsNearbyDuplicate(int[] nums, int k) {
HashMap<Integer,Integer> hm=new HashMap<Integer,Integer>();
for(int i=0;i<nums.length;i++){
if(hm.containsKey(nums[i])&&i-hm.get(nums[i])<=k)
return true;
hm.put(nums[i], i);
}
return false;
}