Merge pull request neetcode-gh#391 from adityadmahale/main

Create 261-Graph-Valid-Tree.java and 252-Meeting-Rooms.java

Author: neetcode-gh

java/252-Meeting-Rooms.java

@@ -0,0 +1,40 @@
+/**
+ * Definition of Interval:
+ * public class Interval {
+ *     int start, end;
+ *     Interval(int start, int end) {
+ *         this.start = start;
+ *         this.end = end;
+ *     }
+ * }
+ */
+
+public class Solution {
+    /**
+     * @param intervals: an array of meeting time intervals
+     * @return: if a person could attend all meetings
+     */
+    public boolean canAttendMeetings(List<Interval> intervals) {
+        int length = intervals.size();
+        if (intervals.size() == 0 || length == 1) return true;
+        // Write your code here
+
+        int[] start = new int[length];
+        int[] end = new int[length];
+        for (int i = 0; i < length; i++) {
+            start[i] = intervals.get(i).start;
+            end[i] = intervals.get(i).end;
+        }
+
+        Arrays.sort(start);
+        Arrays.sort(end);
+
+        int j = 0;
+        while (j + 1 < length) {
+            if (end[j] > start[j + 1]) return false;
+            j++;
+        }
+
+        return true;
+    }
+}

java/261-Graph-Valid-Tree.java

@@ -0,0 +1,40 @@
+public class Solution {
+
+    private Map<Integer, List<Integer>> adjacencyList = new HashMap<>();
+
+    public boolean validTree(int n, int[][] edges) {
+        if (n == 0 || n == 1) return true;
+
+        if (edges.length == 0) return false;
+
+        for (var edge: edges) {
+            var node1 = edge[0];
+            var node2 = edge[1];
+            adjacencyList.putIfAbsent(node1, new ArrayList<>());
+            adjacencyList.putIfAbsent(node2, new ArrayList<>());
+            adjacencyList.get(node1).add(node2);
+            adjacencyList.get(node2).add(node1);
+        }
+
+        Set<Integer> visited = new HashSet<>();
+
+        return depthFirstSearch(0, -1, visited) && visited.size() == n;
+    }
+
+    private boolean depthFirstSearch(int node, int previous, Set<Integer> visited) {
+        if (visited.contains(node)) 
+            return false;
+
+        visited.add(node);
+
+        for (var neighbor: adjacencyList.get(node)) {
+            if (neighbor == previous) 
+                continue;
+
+            if (!depthFirstSearch(neighbor, node, visited))
+                return false;
+        }
+
+        return true;
+    }
+}