547. Friend Circles

Description

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 NN* 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:

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

Solution

Union-Find, time O(n ^ 2), space O(n)

其实就是求连通区的个数。注意连通区初始化为n,而非n ^ 2哦!

class Solution {
    public int findCircleNum(int[][] M) {
        int n = M.length;
        int[] parent = new int[n];
        for (int i = 0; i < parent.length; ++i) {
            parent[i] = i;
        }
        
        int circles = n;    // important
        for (int i = 0; i < n; ++i) {
            for (int j = i + 1; j < n; ++j) {
                if (M[i][j] != 1) {
                    continue;
                }
                
                int iset = find(parent, i);
                int jset = find(parent, j);
                if (iset != jset) {
                    union(parent, iset, jset);
                    --circles;
                }
            }
        }
        
        return circles;
    }
    
    public int find(int[] parent, int x) {
        if (parent[x] != x) {
            parent[x] = find(parent, parent[x]);
        }
        return parent[x];
    }
    
    public void union(int[] parent, int xset, int yset) {
        parent[xset] = yset;
    }
}

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