Two Sum

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

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

Example: Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9, return [0, 1].

Use Hash Map

This method use a hash map to store previous element and its index, so that we can search for candidate in O(1) time.

vector<int> twoSum(vector<int>& nums, int target) {
    unordered_map<int, int> m;
    for(int i=0; i<nums.size(); i++) {
        int candidate = target - nums[i];
        if(m.find(candidate) != m.end()) {
            return {m[candidate], i};
        } else {
            m[nums[i]] = i;
        }
    }

    return {};
}