Contains Duplicate II

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 difference between i and j is at most k.

思路：用hashmap来用空间换时间，就是算每个出现的index跟前面的index距离是否超过k，如果 没有，return true，如果超过，覆盖index，继续往下走。

public class Solution {    public boolean containsNearbyDuplicate(int[] nums, int k) {        if(nums == null || nums.length == 0 || k<=0) return false;        HashMap<Integer,Integer> hashmap = new HashMap<Integer, Integer>();        for(int i=0; i<nums.length; i++){            if(hashmap.containsKey(nums[i])){                if(i - hashmap.get(nums[i]) <=k){                    return true;                }else{                    hashmap.put(nums[i],i);                }            } else {                hashmap.put(nums[i],i);            }        }        return false;    }}