1282. Group the People Given the Group Size They Belong To

There are n people that are split into some unknown number of groups. Each person is labeled with a unique ID from 0 to n - 1.

You are given an integer array groupSizes, where groupSizes[i] is the size of the group that person i is in. For example, if groupSizes[1] = 3, then person 1 must be in a group of size 3.

Return a list of groups such that each person i is in a group of size groupSizes[i].

Each person should appear in exactly one group, and every person must be in a group. If there are multiple answers, return any of them. It is guaranteed that there will be at least one valid solution for the given input.

class Solution {
public:
    vector<vector<int>> groupThePeople(vector<int>& groupSizes) {
        unordered_map<int, vector<int>> lookup;
        vector<vector<int>> res;
        for(int i = 0; i < groupSizes.size(); i++) {
            int sz = groupSizes[i];
            lookup[sz].push_back(i);
            if(lookup[sz].size() == sz) {
                res.push_back(lookup[sz]);
                lookup[sz].clear();
            }
        }
        return res;
    }
};