1. Two Sum

Brute-force

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target)
    {
        int n = nums.size();
        for (int i = 0; i < n; ++i)
        {
            for (int j = i + 1; j < n; ++j)
            {
                if(nums[i] + nums[j] == target) return {i, j};
            }
        }
        return {};
    }
};

• T: $O(N^2)$
• S: $O(1)$

HashMap

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target)
    {
        int n = nums.size();
        unordered_map<int, int> m;
        for (int i = 0; i < n; ++i)
        {
            int complement = target - nums[i];

            if (m.count(complement)) return {i, m[complement]};

            m[nums[i]] = i;
        }
        return {};
    }
};

• T: $O(N)$
• S: $O(N)$