1791. Find Center of Star Graph

Problem Statement

Easy

There is an undirected star graph consisting of n nodes labeled from 1 to n. A star graph is a graph where there is one center node and exactly n - 1 edges that connect the center node with every other node.

You are given a 2D integer array edges where each edges[i] = [u_i, v_i] indicates that there is an edge between the nodes u_i and v_i. Return the center of the given star graph.

Example 1:

Input: edges = [[1,2],[2,3],[4,2]]
Output: 2
Explanation: As shown in the figure above, node 2 is connected to every other node, so 2 is the center.

Example 2:

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

Constraints:

3 <= n <= 10⁵
edges.length == n - 1
edges[i].length == 2
1 <= u_i, v_i <= n
u_i != v_i
The given edges represent a valid star graph.

Java — O(n)

Source file

class Solution {
    public int findCenter(int[][] edges) {
        int n = edges.length;
        Map<Integer, Integer> degree = new HashMap<>();
        for (int i = 0; i < n; i++) {
            degree.put(edges[i][0], degree.getOrDefault(edges[i][0], 0) + 1);
            degree.put(edges[i][1], degree.getOrDefault(edges[i][1], 0) + 1);
        }
        for (Map.Entry<Integer, Integer> e : degree.entrySet())
            if (e.getValue() == n)
                return e.getKey();
        return -1;
    }
}

Java

Source file

class Solution {
    public int findCenter(int[][] edges) {
        int n = edges.length;
        Map<Integer, Integer> degree = new HashMap<>();
        for (int i = 0; i < n; i++) {
            degree.put(edges[i][0], degree.getOrDefault(edges[i][0], 0) + 1);
            degree.put(edges[i][1], degree.getOrDefault(edges[i][1], 0) + 1);
        }
        for (Map.Entry<Integer, Integer> e : degree.entrySet())
            if (e.getValue() == n)
                return e.getKey();
        return -1;
    }
}