Sum

Summation problem is a classic interview question with high frequency. For example, 2Sum, 3Sum, 4Sum.

2Sum:

Two Sum problem is the basic problem of all other problems. And
Two Sum Problem Analysis 1: Sort and Hash with Unique Solution
Two Sum Problem Analysis 2: Sort and Hash with Not Unique and No Duplicates
Two Sum Problem Analysis 3: Sort and Hash with not Unique and Duplicates