Contain 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.

Analyse: 

1. For every element in the array, find if other elements meet its requirement.

    Time Limited Exceeded.

class Solution {
public:
    bool containsNearbyDuplicate(vector<int>& nums, int k) {
        if(nums.size() <= 1) return 0 <= k;
        
        for(int i = 0; i < nums.size(); i++){
            for(int j = i + 1; j <= i + k; j++){
                if(nums[j] == nums[i]) return true;
            }
        }
        return false;
    }
};

2. As long as one of the element in the remaining part satisfy the condition, we can return true.

    Runtime: 32ms.

class Solution {
public:
    bool containsNearbyDuplicate(vector<int>& nums, int k) {
        unordered_map<int, int> up;
        for(int i = 0; i < nums.size(); i++){
            if(up.find(nums[i]) != up.end() && i - up.find(nums[i])->second <= k) return true;
            else up[nums[i]] = i;
        }
        return false;
    }
};