Two Sum

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

You can return the answer in any order.

O(N^2) Time Constant space

• Brute force - check every pair of numbers.

Code

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        vector<int> ans;
        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){
                    ans.push_back(i);
                    ans.push_back(j);
                }
            }
        }
        return ans;
    }
};

O(N) Time and O(N) space

• The basic idea is to maintain a hash table for each element num in nums
• using num as key and its index (0-based) as value. For each num, search for target - num in the hash table.
• If it is found and is not the same element as num, then we are done.

Code

class Solution{
public:
    vector<int> twoSum(vector<int> &nums, int target)
    {
        int n = nums.size();
        unordered_map<int, int> mp;
        for (int i = 0; i < n; i++)
        {
            int num = target - nums[i];
            if (mp.find(num) != mp.end()) return {i, mp[num]};
            mp[nums[i]] = i;
        }
        return {};
    }
};