157.Contains Duplicate II

题目：

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.

给定整数数组和整数k，找出数组中是否存在两个不同的索引i和j，使得nums [i] = nums [j]并且i和j之间的绝对差值最多为k。

Example 1:

Input: nums = [1,2,3,1], k = 3
Output: true

Example 2:

Input: nums = [1,0,1,1], k = 1
Output: true

Example 3:

Input: nums = [1,2,3,1,2,3], k = 2
Output: false

解答：

方法一：建立hashmap值和位置的映射

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

方法二：hashset无法增加新的值时，说明有重复元素

class Solution {
    public boolean containsNearbyDuplicate(int[] nums, int k) {
        Set set=new HashSet();
        for(int i=0;i<nums.length;i++){
            if(i>k) set.remove(nums[i-k-1]);
            if(!set.add(nums[i])) return true;
        }
        return false;
    }
}

详解：