1. Two Sum – Solution

Difficulty: Easy
Topics: Array, Hash Table

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.

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]

Solution

While storing the numbers in hashmap, check if the complementary (target – number) is in the hashmap.

Note that hashmap[nums[i]] = i; should not be before the part where we search for comp. This is because search for comp should be done in values previously put in, not including the current value.

vector<int> twoSum(vector<int>& nums, int target) {
        unordered_map<int, int> hashmap;
        vector<int> res;
        for (int i = 0; i < nums.size(); i++){
            int comp = target - nums[i];
            if (hashmap.count(comp) > 0){
                res = {hashmap[comp], i};
                return res;
            }
            hashmap[nums[i]] = i;
        }
        return res;
    }