219. Contains Duplicate II

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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.

解法1：O(N) Time + O(N) Space

用一个HashMap来存储每一个number出现的位置。然后对于所有出现过两次以上的数字计算是否有满足的答案。
Java

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public class Solution {
    public boolean containsNearbyDuplicate(int[] nums, int k) {
        
        if (nums == null || nums.length == 0) {
            return false;
        }
        
        Map<Integer, List<Integer>> map = new HashMap<Integer, List<Integer>>();
        for (int i = 0; i < nums.length; ++i) {
            if (map.containsKey(nums[i])) {
                map.get(nums[i]).add(i);
            } else {
                map.put(nums[i], new ArrayList<Integer>());
                map.get(nums[i]).add(i);
            }
        }
        
        // traverse the HashMap
        for (int num : map.keySet()) {
            if (map.get(num).size() >= 2) {
                List<Integer> indices = map.get(num);
                // traverse the list
                for (int i = 0; i < indices.size() - 1; ++i) {
                    if (indices.get(i + 1) - indices.get(i) <= k) {
                        return true;
                    }
                }
            }
        }
        
        return false;
    }
}