AC python union find


  • 0
    A

    video link for union find explanation: https://youtu.be/ID00PMy0-vE

    class Solution(object):
        def validTree(self, n, edges):
            """
            :type n: int
            :type edges: List[List[int]]
            :rtype: bool
            """
            ds = DisjointSet()
            
            # make set for each node
            for i in range(n):
                ds.make_set(i)
                
            for from_vert, to_vert in edges:
                parent1 = ds.find_set(from_vert)
                parent2 = ds.find_set(to_vert)
                
                # if two nodes have the same parent return False
                if parent1 == parent2:
                    return False
                ds.union(from_vert, to_vert) # union nodes with different parents
            
            # in the end number of sets should equal 1
            return ds.num_sets == 1
    
    
    class Node(object):
        def __init__(self, data, parent = None, rank = 0):
            self.data = data
            self.parent = parent
            self.rank = rank
    
        def __str__(self):
            return str(self.data)
    
        def __repr__(self):
            return self.__str__()
    
    
    class DisjointSet(object):
        def __init__(self):
            self.map = {}
            self.num_sets = 0
    
        def make_set(self, data):
            node = Node(data)
            node.parent = node # very important!
            self.map[data] = node
            self.num_sets += 1 # make_set increases the number of disjoint sets by one
    
        def union(self, data1, data2):
            # gets nodes given data values
            node1 = self.map[data1]
            node2 = self.map[data2]
    
            # get parents given nodes
            parent1 = self.find_set_util(node1)
            parent2 = self.find_set_util(node2)
    
            # if they are part of same set do nothing
            if parent1.data == parent2.data:
                return
    
            # else whoever's rank is higher becomes parent of other
            if parent1.rank >= parent2.rank:
                # increment rank only if both sets have same rank
                if parent1.rank == parent2.rank:
                    parent1.rank = parent1.rank + 1
                parent2.parent = parent1
            else:
                parent1.parent = parent2
    
            self.num_sets -= 1 # union decreases the number of disjoint sets by one
    
        # Finds the representative of this set
        def find_set(self, data):
            return self.find_set_util(self.map[data]) # pass in the node
    
        # Find the representative recursively and does path compression as well.
        def find_set_util(self, node):
            parent = node.parent
            if parent == node:
                return parent
    
            node.parent = self.find_set_util(node.parent) # path compression
            return node.parent
            
    

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