1. Two sum — Leetcode (easy) #1

Hi guys, today I will talk about ONE OF THE EASIEST PROBLEMS in LeetCode. It is Two Sum problem. Let’s start.

1. First, we have to look question.

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.

Cool. They gave us 3 examples, too.

Example 1:

Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].

Example 2:

Input: nums = [3,2,4], target = 6
Output: [1,2]

Example 3:

Input: nums = [3,3], target = 6
Output: [0,1]

Constraints:

• 2 <= nums.length <= 104
• -109 <= nums[i] <= 109
• -109 <= target <= 109
• Only one valid answer exists.

Follow-up: Can you come up with an algorithm that is less than O(n2) time complexity?

HASHMAP METHOD:

class Solution {
public:
vector<int> twoSum(vector<int>& nums, int target) {
vector<int>ans;
unordered_map<int,int>m;

for(int i=0;i<nums.size();i++)
{
int val = target-nums[i];
if(m.find(val)!=m.end()) // in case the second element is found
{
ans.push_back(m.find(val)->second);
ans.push_back(i);
break;
}
m.insert(pair<int,int>(nums[i],i));
}
return ans;
}
};