323. 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.

``````class Solution {
public:
int countComponents(int n, vector<pair<int, int> >& edges) {
int res = 0;
vector<vector<int> > g(n);
vector<bool> v(n, false);
for (auto a : edges) {
g[a.first].push_back(a.second);
g[a.second].push_back(a.first);
}
for (int i = 0; i < n; ++i) {
if (!v[i]) {
++res;
dfs(g, v, i);
}
}
return res;
}
void dfs(vector<vector<int> > &g, vector<bool> &v, int i) {
if (v[i]) return;
v[i] = true;
for (int j = 0; j < g[i].size(); ++j) {
dfs(g, v, g[i][j]);
}
}
};
``````

``````class Solution {
public:
int countComponents(int n, vector<pair<int, int> >& edges) {
int res = n;
vector<int> root(n);
for (int i = 0; i < n; ++i) root[i] = i;
for (auto a : edges) {
int x = find(root, a.first), y = find(root, a.second);
if (x != y) {
--res;
root[y] = x;
}
}
return res;
}
int find(vector<int> &root, int i) {
while (root[i] != i) i = root[i];
return i;
}
};
``````

Clone Graph

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https://leetcode.com/discuss/77308/accepted-dfs-in-c

https://leetcode.com/discuss/77027/c-solution-using-union-find

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

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