Map with a vector as a parameter [closed] - c++

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For the Leetcode:
There are n people whose IDs go from 0 to n - 1 and each person belongs exactly to one group. Given the array groupSizes of length n telling the group size each person belongs to, return the groups there are and the people's IDs each group includes.
You can return any solution in any order and the same applies for IDs. Also, it is guaranteed that there exists at least one solution.
Example 1:
Input: groupSizes = [2,1,3,3,3,2]
Output: [[1],[0,5],[2,3,4]]
class Solution {
public:
vector<vector<int>> groupThePeople(vector<int>& groupSizes) {
unordered_map<int,vector<int> > myMap;
int n=groupSizes.size();
vector<vector<int>> answer;
for(int i=0;i<n;i++){
myMap[groupSizes[i]].push_back(i); // myMap key/value ; key= group, value=index
cout<<i<<endl;
if(myMap[groupSizes[i]].size()==groupSizes[i]){
cout<<"pushed "<<i<<endl;
answer.push_back(myMap[groupSizes[i]]);
myMap[groupSizes[i]]={};
}
}
return answer;
}
};
Does the map contain a bunch of different vectors, or is there only 1 vector?
Could you explain what exactly is being pushed? When you have map<int,vector<int>>; are you pushing the groupsize as the key, and then the value is the index?
Thus, would the map look like map[groupsize value, vector of indexes]?
How did the output get its first vector [1]? If the value 2 should of been pushed the vector first?

Answer 1

Does the map contain a bunch of different vectors, or is there only 1
vector?
For each key in your map, there exists one vector as a value. In your example, there are 3 vectors:
Key "1", Vector ( 1 )
Key "2", Vector ( 0, 5 )
Key "3", Vector ( 2, 3, 4 )
Could you explain what exactly is being pushed? When you have
map<int,vector<int>>; are you pushing the groupsize as the key, and
then the value is the index?
What you are pushing and where is dependent on the current value of i and n. n is set to the size of groupSizes, so 6. i ranges from 0 to 5. On the first iteration, push_back is called the following way:
myMap[groupSizes[0]].push_back(0);
groupSizes[0] has value "2", as the index 0 for [2,1,3,3,3,2] is "2"
myMap[groupSizes[0]] searches for value "2" in the map. If the key does not exist yet, it is inserted. It then returns the value; if it did not exist yet, it is created -> an empty vector will be returned.
push_back(0) the value "0" is added to the vector for value "2"
Thus, would the map look like map[groupsize value, vector of indexes]?
Yes, but note that the groupsize values are grouped within the map. There are only the values 1, 2 and 3 in it and not 2, 1, 3, 3, 3 and 2.
How did the output get its first vector [1]? If the value 2 should of
been pushed the vector first?
I think you final output code is missing, but I would guess that your output just iterates over your result vector. If that is the case, the order is just a happy coincidence. The groupSize value tells you, how many elements there will be in the vector. So the lower the groupSize value, the sooner it will be finished and pushed into the result vector. So "1" only contains one element and will be pushed first into the result vector, as if(myMap[groupSizes[i]].size()==groupSizes[i]).