Solve countComponents

Author: bky373

number-of-connected-components-in-an-undirected-graph/bky373.java

@@ -0,0 +1,42 @@
+/*
+    time: O(n+m), where n is the number of nodes and m is the number of edges in the graph.
+    space: O(n)
+ */
+class Solution {
+
+    public int countComponents(int n, int[][] edges) {
+        boolean[] visited = new boolean[n];
+
+        Map<Integer, List<Integer>> map = new HashMap<>();
+        for (int[] edge : edges) {
+            map.computeIfAbsent(edge[0], k -> new ArrayList<>())
+               .add(edge[1]);
+            map.computeIfAbsent(edge[1], k -> new ArrayList<>())
+               .add(edge[0]);
+        }
+
+        int ans = 0;
+        for (int i = 0; i < n; i++) {
+            if (visited[i]) {
+                continue;
+            }
+            dfs(i, map, visited);
+            ans++;
+        }
+        return ans;
+    }
+
+    public void dfs(int k, Map<Integer, List<Integer>> map, boolean[] visited) {
+        if (visited[k]) {
+            return;
+        }
+        visited[k] = true;
+        if (!map.containsKey(k)) {
+            return;
+        }
+        List<Integer> values = map.get(k);
+        for (int v : values) {
+            dfs(v, map, visited);
+        }
+    }
+}