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requirements: You must have a good understanding of containers, classes, member functions, and the structure dereference operator like this " -> " for this problem 1. You need to first understand that maps store data as (key, value) pairs. The "Value" is also called "second" because "key" is "first" (key, VALUE) == (first, SECOND) 2. Second you need to understand that "unordered_map m" begins life uninitialized == no data. 3. Third "unordered_map m" only job is to just hold unique values. Some values in some address of map m -> "second (value)" will not be used. data member accessing example via dealing with the object directly or accessing object via a pointer. - Object directly method: m.someMemeberFunction( ) // this accesses a public member function - pointer to a object has to be dereferenced like this (*m).someMemeberFunciton( ) or using the arrow m->someMemberFunction( ) the arrow is preferred for readability. 4. unordered_map m initialization: "unordered_map m" job here is to be initialized via push_back (aka "appended to end") ONLY if the "if (statement)" was false, and is initialized as follows: ( key = nums[ i ] , value = int " i " ) where int " i " represents the index number where you can find the value of nums[ i ]. Loop { Iteration 0 of the for loop will skip the if statement because m.find(diff) will be false because the unordered_map is uninitialized so find(diff) will return m.end(). This will proceed to initialize the map in the way i explained above. diff = target - nums[i] Remember this. if ( m.find(diff) != m.end() // if the diff (difference) of target - nums[ i ] was previously initialized inside " m " this implies a match exists so you push m-> second back // and " i " && if ( m.find(diff) -> second != i // this statement is making sure there are only unique instances stored in map "m" do: { push_back( i ) to the vector "ret" (return vector) because int " i " represents the subscript index number to a number that meets the conditions of the twoSum problem push_back(m.find(diff)->second) takes the address returned from the find(diff) function call and dereferences the class and accesses the data member second which in this case is the "value" of the (key, value) pair of our map "m". } check out this resource for everything you could ever want to know about the STL (standard template library) cplusplus.com/reference/vector/vector/push_back/