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Find Champion II

MediumUpdated: Aug 2, 2025
Practice on:
LeetCode

Problem

There are n teams numbered from 0 to n - 1 in a tournament; each team is also a node in a DAG.

You are given the integer n and a 0-indexed 2D integer array edges of length m representing the DAG , where edges[i] = [ui, vi] indicates that there is a directed edge from team ui to team vi in the graph.

A directed edge from a to b in the graph means that team a is stronger than team b and team b is weaker than team a.

Team a will be the champion of the tournament if there is no team b that is stronger than team a.

Return _the team that will be thechampion of the tournament if there is a unique champion, otherwise, return _-1 .

Notes

  • A cycle is a series of nodes a1, a2, ..., an, an+1 such that node a1 is the same node as node an+1, the nodes a1, a2, ..., an are distinct, and there is a directed edge from the node ai to node ai+1 for every i in the range [1, n].
  • A DAG is a directed graph that does not have any cycle.

Examples

Example 1

graph TB;
	0 --> 1 
	1 --> 2

Input: n = 3, edges = [[0,1],[1,2]]
Output: 0
Explanation: Team 1 is weaker than team 0. Team 2 is weaker than team 1. So the champion is team 0.

Example 2

graph TB;
	0 --> 2
	1 --> 2
	1 --> 3

Input: n = 4, edges = [[0,2],[1,3],[1,2]]
Output: -1
Explanation: Team 2 is weaker than team 0 and team 1. Team 3 is weaker than team 1. But team 1 and team 0 are not weaker than any other teams. So the answer is -1.

Constraints

  • 1 <= n <= 100
  • m == edges.length
  • 0 <= m <= n * (n - 1) / 2
  • edges[i].length == 2
  • 0 <= edge[i][j] <= n - 1
  • edges[i][0] != edges[i][1]
  • The input is generated such that if team a is stronger than team b, team b is not stronger than team a.
  • The input is generated such that if team a is stronger than team b and team b is stronger than team c, then team a is stronger than team c.

Solution

Method 1 - Using indegrees

Here is the approach:

  1. Understanding the Problem:
    • Each team is a node in a Directed Acyclic Graph (DAG).
    • A directed edge from node a to node b indicates team a is stronger than team b.
  2. Goal:
    • Determine if there is a unique champion i.e., a node with no incoming edges except itself.
  3. Plan:
    • Initialize an array to track the in-degree of each node.
    • Traverse the graph and compute the in-degree for each node.
    • Check for nodes with zero in-degree. There should be exactly one such node for a unique champion.

Code

Java
class Solution {
    public int findChampion(int n, int[][] edges) {
        int[] inDegree = new int[n];
        
        // Calculate in-degrees
        for (int[] edge : edges) {
            inDegree[edge[1]]++;
        }
        
        int champion = -1;
        
        // Look for the node with in-degree 0
        for (int i = 0; i < n; i++) {
            if (inDegree[i] == 0) {
                if (champion == -1) {
                    champion = i;
                } else {
                    // More than one node with in-degree 0
                    return -1;
                }
            }
        }
        
        return champion;
    }
}
Python
class Solution:
    def findChampion(self, n: int, edges: List[List[int]]) -> int:
        in_degree: List[int] = [0] * n
        
        # Calculate in-degrees
        for edge in edges:
            in_degree[edge[1]] += 1
        
        champion: int = -1
        
        # Look for the node with in-degree 0
        for i in range(n):
            if in_degree[i] == 0:
                if champion == -1:
                    champion = i
                else:
                    # More than one node with in-degree 0
                    return -1
                
        return champion

Complexity

  • ⏰ Time complexity: O(n + m)
    • O(n) for initializing the array.
    • O(m) for traversing the edges.
  • 🧺 Space complexity: O(n) for the in-degree array.

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