Formatted question description: https://leetcode.ca/all/1791.html

1791. Find Center of Star Graph

Level

Medium

Description

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 <= 10^5
• 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.

Solution

The given star graph is actually a tree, with n nodes and n - 1 edges. The center of the star graph has n - 1 edges connected to it, and each of the remaining nodes has 1 edge connected to it. Simply calculate the number of edges connected to each node, and return the node that has more than 1 edge connected to it.

• Java
• C++
• Python

• class Solution {
      public int findCenter(int[][] edges) {
          int n = edges.length + 1;
          int[] connections = new int[n + 1];
          for (int[] edge : edges) {
              int node1 = edge[0], node2 = edge[1];
              connections[node1]++;
              connections[node2]++;
              if (connections[node1] > 1)
                  return node1;
              else if (connections[node2] > 1)
                  return node2;
          }
          return 0;
      }
  }
  

• // OJ: https://leetcode.com/problems/find-center-of-star-graph/
  // Time: O(1)
  // Space: O(1)
  class Solution {
  public:
      int findCenter(vector<vector<int>>& E) {
          return E[0][0] == E[1][0] || E[0][0] == E[1][1] ? E[0][0] : E[0][1];
      }
  };
  

• # 1791. Find Center of Star Graph
  # https://leetcode.com/problems/find-center-of-star-graph/
  
  class Solution:
      def findCenter(self, edges: List[List[int]]) -> int:
          n = len(edges)
          mp = collections.defaultdict(int)
          
          for a,b in edges:
              mp[a] += 1
              mp[b] += 1
          
          for key in mp:
              if mp[key] == n:
                  return key
          
          return -1
          
  
  

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