add graph valid tree solution

Author: Tessa1217

graph-valid-tree/Tessa1217.java

@@ -0,0 +1,57 @@
+public class Solution {
+    /**
+     * @param n: An integer
+     * @param edges: a list of undirected edges
+     * @return: true if it's a valid tree, or false
+     */
+    // 시간 복잡도: O(n), 공간복잡도: O(n)
+    public boolean validTree(int n, int[][] edges) {
+      
+        if (edges.length != (n - 1)) {
+            return false; 
+        }
+
+        List<Integer>[] graph = new ArrayList[n];
+        for (int i = 0; i < n; i++) {
+            graph[i] = new ArrayList<>();
+        }
+
+        for (int[] edge : edges) {
+            graph[edge[0]].add(edge[1]);
+            graph[edge[1]].add(edge[0]);
+        }
+
+        boolean[] visited = new boolean[n];
+
+        if (!dfs(0, -1, visited, graph)) {
+            return false;
+        }
+
+        for (boolean v : visited) {
+            if (!v) {
+                return false; // Not fully connected
+            }
+        }
+
+        return true;
+    }
+
+    private boolean dfs(int node, int parent, boolean[] visited, List<Integer>[] graph) {
+        if (visited[node]) {
+            return false; // Found a cycle
+        }
+
+        visited[node] = true;
+
+        for (int neighbor : graph[node]) {
+            if (neighbor != parent) {
+                if (!dfs(neighbor, node, visited, graph)) {
+                    return false;
+                }
+            }
+        }
+
+        return true;
+    }
+}
+