[LeetCode 219] Contains Duplicat

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.

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

Solution: HashMap

1. 只需要记录每个数字的index，一旦出现重复得数字，然后比较两个index之间的距离即可。因为不要求 == k，只要求不大于k，所以不需要记录全部index

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