1. Two Sum

• Difficulty: Easy

• Topics: Array, Hash Table

• Similar Questions:

  • 3Sum

  • 4Sum

  • Two Sum II - Input array is sorted

  • Two Sum III - Data structure design

  • Subarray Sum Equals K

  • Two Sum IV - Input is a BST

  • Two Sum Less Than K

Problem:

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].

Solutions:

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

        for (int i = 0; i < nums.size(); ++i) {
            if (numToIndex.count(target - nums[i]) > 0) {
                return {numToIndex[target - nums[i]], i};
            }
            numToIndex[nums[i]] = i;
        }

        return {};
    }
};