LeetCode in Python 547. Friend Circles

There are N students in a class. Some of them are friends, while some are not. Their friendship is transitive in nature. For example, if A is a direct friend of B, and B is a direct friend of C, then A is an indirect friend of C. And we defined a friend circle is a group of students who are direct or indirect friends.

Given a N*N matrix M representing the friend relationship between students in the class. If M[i][j] = 1, then the ith and jth students are direct friends with each other, otherwise not. And you have to output the total number of friend circles among all the students.

Example 1:

Input: 
[[1,1,0],
 [1,1,0],
 [0,0,1]]
Output: 2
Explanation:The 0th and 1st students are direct friends, so they are in a friend circle. 
The 2nd student himself is in a friend circle. So return 2.

Example 2:

Input: 
[[1,1,0],
 [1,1,1],
 [0,1,1]]
Output: 1
Explanation:The 0th and 1st students are direct friends, the 1st and 2nd students are direct friends, 
so the 0th and 2nd students are indirect friends. All of them are in the same friend circle, so return 1.

Note:

N is in range [1,200].
M[i][i] = 1 for all students.
If M[i][j] = 1, then M[j][i] = 1.

Solution:

class Solution(object):
    def findCircleNum(self, M):
        """
        :type M: List[List[int]]
        :rtype: int
        """
        #map: key:# value:group#
        #list[i] => i-th element's group#
        rows = len(M)
        groupMap = [i for i in range(rows)]
        
        def dfs(group, index):
            groupMap[index] = group
            for col, friend in enumerate(M[index]):
                if col != index and friend and groupMap[col] == col:
                    dfs(group, col)
            
        for row in range(rows):
            if groupMap[row] != row: continue
            dfs(row, row)
                    
        return len(set(groupMap))

dfs title famous circle of friends, the idea is the serial number starts from 0, find all its friends up (similar to FIG communication), these people the same Group, to list instead of the map represents the i-th element of the corresponding group number, to the end it takes the set, i.e. the number of the longitudinal circle of friends.

LeetCode in Python 547. Friend Circles

Guess you like