problemmediumalgorithmsleetcode-1282leetcode 1282leetcode1282

Group the People Given the Group Size They Belong To

MediumUpdated: Aug 2, 2025
Practice on:
LeetCode

Problem

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.

Examples

Example 1

Input: groupSizes = [3,3,3,3,3,1,3]
Output: [[5],[0,1,2],[3,4,6]]
Explanation: 
The first group is [5]. The size is 1, and groupSizes[5] = 1.
The second group is [0,1,2]. The size is 3, and groupSizes[0] = groupSizes[1] = groupSizes[2] = 3.
The third group is [3,4,6]. The size is 3, and groupSizes[3] = groupSizes[4] = groupSizes[6] = 3.
Other possible solutions are [[2,1,6],[5],[0,4,3]] and [[5],[0,6,2],[4,3,1]].

Example 2

Input: groupSizes = [2,1,3,3,3,2]
Output: [[1],[0,5],[2,3,4]]

Constraints

  • groupSizes.length == n
  • 1 <= n <= 500
  • 1 <= groupSizes[i] <= n

Solution

Method 1 – Hash Map Grouping and Greedy Assignment

Intuition

We can use a hash map to collect people by their required group size. As soon as we collect enough people for a group, we finalize that group and start a new one. This greedy approach ensures all constraints are satisfied.

Approach

  1. Initialize a hash map to store lists of people for each group size.
  2. Iterate through groupSizes, for each person i:
    • Add i to the list for groupSizes[i].
    • If the list reaches the required group size, add it to the answer and reset the list.
  3. Return the list of groups.

Code

C++
class Solution {
public:
    vector<vector<int>> groupThePeople(vector<int>& groupSizes) {
        unordered_map<int, vector<int>> mp;
        vector<vector<int>> ans;
        for (int i = 0; i < groupSizes.size(); ++i) {
            mp[groupSizes[i]].push_back(i);
            if (mp[groupSizes[i]].size() == groupSizes[i]) {
                ans.push_back(mp[groupSizes[i]]);
                mp[groupSizes[i]].clear();
            }
        }
        return ans;
    }
};
Go
func groupThePeople(groupSizes []int) [][]int {
    mp := map[int][]int{}
    var ans [][]int
    for i, sz := range groupSizes {
        mp[sz] = append(mp[sz], i)
        if len(mp[sz]) == sz {
            ans = append(ans, append([]int{}, mp[sz]...))
            mp[sz] = nil
        }
    }
    return ans
}
Java
class Solution {
    public List<List<Integer>> groupThePeople(int[] groupSizes) {
        Map<Integer, List<Integer>> mp = new HashMap<>();
        List<List<Integer>> ans = new ArrayList<>();
        for (int i = 0; i < groupSizes.length; i++) {
            int sz = groupSizes[i];
            mp.computeIfAbsent(sz, k -> new ArrayList<>()).add(i);
            if (mp.get(sz).size() == sz) {
                ans.add(new ArrayList<>(mp.get(sz)));
                mp.get(sz).clear();
            }
        }
        return ans;
    }
}
Kotlin
class Solution {
    fun groupThePeople(groupSizes: IntArray): List<List<Int>> {
        val mp = mutableMapOf<Int, MutableList<Int>>()
        val ans = mutableListOf<List<Int>>()
        for (i in groupSizes.indices) {
            val sz = groupSizes[i]
            mp.getOrPut(sz) { mutableListOf() }.add(i)
            if (mp[sz]!!.size == sz) {
                ans.add(mp[sz]!!.toList())
                mp[sz]!!.clear()
            }
        }
        return ans
    }
}
Python
class Solution:
    def groupThePeople(self, groupSizes: list[int]) -> list[list[int]]:
        from collections import defaultdict
        mp: dict[int, list[int]] = defaultdict(list)
        ans: list[list[int]] = []
        for i, sz in enumerate(groupSizes):
            mp[sz].append(i)
            if len(mp[sz]) == sz:
                ans.append(mp[sz][:])
                mp[sz].clear()
        return ans
Rust
impl Solution {
    pub fn group_the_people(group_sizes: Vec<i32>) -> Vec<Vec<i32>> {
        use std::collections::HashMap;
        let mut mp: HashMap<i32, Vec<i32>> = HashMap::new();
        let mut ans = Vec::new();
        for (i, &sz) in group_sizes.iter().enumerate() {
            let e = mp.entry(sz).or_insert(Vec::new());
            e.push(i as i32);
            if e.len() == sz as usize {
                ans.push(e.clone());
                e.clear();
            }
        }
        ans
    }
}
TypeScript
class Solution {
    groupThePeople(groupSizes: number[]): number[][] {
        const mp: Record<number, number[]> = {};
        const ans: number[][] = [];
        for (let i = 0; i < groupSizes.length; i++) {
            const sz = groupSizes[i];
            if (!mp[sz]) mp[sz] = [];
            mp[sz].push(i);
            if (mp[sz].length === sz) {
                ans.push([...mp[sz]]);
                mp[sz] = [];
            }
        }
        return ans;
    }
}

Complexity

  • ⏰ Time complexity: O(n) — Each person is processed once.
  • 🧺 Space complexity: O(n) — For storing group assignments.

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