Contains Duplicate II

Description

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

Solutions

My solution

class Solution {
public:
    bool containsNearbyDuplicate(vector<int>& nums, int k) {
        unordered_map<int, int> hash; // hash[num]: frequency of num in the sliding window of length k
        for(int i = 0; i < nums.size(); ++i){
            if(++hash[nums[i]] > 1)
                return true;
            if(i >= k)
                --hash[nums[i - k]];
        }
        return false;
    }
};

Another solution

class Solution {
public:
    bool containsNearbyDuplicate(vector<int>& nums, int k) {
        unordered_map<int, int> hash; // hash[num]: the index of the last occurrence of num
        for(int i = 0; i < nums.size(); ++i){
            auto it = hash.find(nums[i]);
            if(it != hash.end() && i - it->second <= k)
                return true;
            hash[nums[i]] = i;
        }
        return false;
    }
};