find champion ii.py

'''
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 the champion 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.
'''


class Solution:
    def findChampion(self, n: int, edges: List[List[int]]) -> int:
        g = defaultdict(list)

        #check for nodes that have incoming edges
        for a,b in edges:
            g[b].append(a)
        
        
        res = []

        for i in range(n):
            if g[i] == []:
                res.append(i)
        if len(res) > 1:
            return -1
        return res[0]