[leetcode] 323. Number of Connected Components in an Undirected Graph 解题报告

题目链接:https://leetcode.com/problems/number-of-connected-components-in-an-undirected-graph/

Given n nodes labeled from 0 to n - 1 and a list of undirected edges (each edge is a pair of nodes), write a function to find the number of connected components in an undirected graph.

Example 1:

     0          3
     |          |
     1 --- 2    4

Given n = 5 and edges = [[0, 1], [1, 2], [3, 4]], return 2.

Example 2:

     0           4
     |           |
     1 --- 2 --- 3

Given n = 5 and edges = [[0, 1], [1, 2], [2, 3], [3, 4]], return 1.

Note:
You can assume that no duplicate edges will appear in edges. Since all edges are undirected, [0, 1] is the same as [1, 0] and thus will not appear together in edges.

思路: 为每一个结点设置一个父结点, 初始为本身. 如果一条边的最终父结点相同, 则说明他们是联通的, 否则就将第二个结点的最终父结点设置为第一个结点的最终父结点.

这样初始总共有n个不联通的结点, 每次新添加一条边, 如果他们最终父结点不想同, 则说明减少一个不联通的点.

代码如下:

class Solution {
public:
    int countComponents(int n, vector<pair<int, int>>& edges) {
        vector<int> parent(n);
        int ans = n;
        for(int i = 0; i< n; i++) parent[i] = i;
        for(auto val: edges)
        {
            int pa = val.first, pb = val.second;
            while(parent[pa] != pa) pa = parent[pa]; 
            while(parent[pb] != pb) pb = parent[pb]; 
            if(pa != pb)
            {
                parent[pb] = pa;
                ans--;
            }
        }
        return ans;
    }
};

参考: https://leetcode.com/discuss/76519/similar-to-number-of-islands-ii-with-a-findroot-function