Working on the following problem (https://leetcode.com/problems/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. Ex:
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.
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.
Here's my solution:
class Solution(object):
def findCircleNum(self, M):
"""
:type M: List[List[int]]
:rtype: int
"""
parents = [i for i in range(len(M))]
count = len(M)
def union(i, j):
parent_i = get_parent(i)
parent_j = get_parent(j)
parents[i] = parent_j
def get_parent(i):
while not parents[i] == i:
parents[i] = parents[parents[i]] # compress
i = parents[i]
return i
for i in range(len(M)):
for j in range(i+1, len(M)):
if M[i][j] == 1:
union(i, j)
return sum(i == parent for i, parent in enumerate(parents))
This code breaks for the following input:
[
[1,0,0,0,0,0,0,0,0,1,0,0,0,0,0],
[0,1,0,1,0,0,0,0,0,0,0,0,0,1,0],
[0,0,1,0,0,0,0,0,0,0,0,0,0,0,0],
[0,1,0,1,0,0,0,1,0,0,0,1,0,0,0],
[0,0,0,0,1,0,0,0,0,0,0,0,1,0,0],
[0,0,0,0,0,1,0,0,0,0,0,0,0,0,0],
[0,0,0,0,0,0,1,0,0,0,0,0,0,0,0],
[0,0,0,1,0,0,0,1,1,0,0,0,0,0,0],
[0,0,0,0,0,0,0,1,1,0,0,0,0,0,0],
[1,0,0,0,0,0,0,0,0,1,0,0,0,0,0],
[0,0,0,0,0,0,0,0,0,0,1,0,0,0,0],
[0,0,0,1,0,0,0,0,0,0,0,1,0,0,0],
[0,0,0,0,1,0,0,0,0,0,0,0,1,0,0],
[0,1,0,0,0,0,0,0,0,0,0,0,0,1,0],
[0,0,0,0,0,0,0,0,0,0,0,0,0,0,1]
]
(my solution returns 10 instead of 8) and I'm having a bit of trouble tracing where my algorithm is incorrect. Does anyone see anything wrong here? NB: It's wrapped in a class Solution because this is a Leetcode thing.
You wrote parents[i] = parent_j instead of parents[parent_i] = parent_j, allowing the possibility of moving object i into the set parent_j without bringing the rest of its set.