[Leo] 9th Week solutions (rest 3Qs)

clone-graph/Leo.py

@@ -0,0 +1,20 @@
+class Solution:
+    def cloneGraph(self, node: 'Node') -> 'Node':
+        oldToNew = {}
+
+        def dfs(node):
+            if node in oldToNew:
+                return oldToNew[node]
+
+            copy = Node(node.val)
+            oldToNew[node] = copy
+
+            for nei in node.neighbors:
+                copy.neighbors.append(dfs(nei))
+
+            return copy
+
+        return dfs(node) if node else None
+
+        ## TC: O(num(node) + num(edge))
+        ## SC: O(num(node))

course-schedule/Leo.py

@@ -0,0 +1,27 @@
+class Solution:
+    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
+        indegree = collections.defaultdict(int)
+        adj_list = collections.defaultdict(list)
+
+        for pre in prerequisites:
+            indegree[pre[0]] += 1
+            adj_list[pre[1]].append(pre[0])
+
+        starts = deque([i for i in range(numCourses) if indegree[i] == 0])
+        visited = set()
+
+        for start in starts:
+            self.dfs(indegree, adj_list, start, visited)
+
+        return len(visited) == numCourses
+
+    def dfs(self, indegree, adj_list, start, visited):
+        if start in visited:
+            return
+        visited.add(start)
+        for neigh in adj_list[start]:
+            indegree[neigh] -= 1
+            if indegree[neigh] == 0:
+                self.dfs(indegree, adj_list, neigh, visited)
+
+    ## TC & SC: O(num(node)+num(edge))

pacific-atlantic-water-flow/Leo.py

@@ -0,0 +1,29 @@
+class Solution:
+    def pacificAtlantic(self, heights: List[List[int]]) -> List[List[int]]:
+        if not heights or not heights[0]:
+            return []
+
+        m, n = len(heights), len(heights[0])
+        p_visited = set()
+        a_visited = set()
+        directions = [(-1, 0), (1, 0), (0, 1), (0, -1)]
+
+        def dfs(visited, x, y):
+            visited.add((x, y))
+            for dx, dy in directions:
+                new_x, new_y = x + dx, y + dy
+                if 0 <= new_x < m and 0 <= new_y < n and (new_x, new_y) not in visited and heights[new_x][new_y] >= \
+                        heights[x][y]:
+                    dfs(visited, new_x, new_y)
+
+        for i in range(m):
+            dfs(p_visited, i, 0)
+            dfs(a_visited, i, n - 1)
+
+        for j in range(n):
+            dfs(p_visited, 0, j)
+            dfs(a_visited, m - 1, j)
+
+        return list(p_visited.intersection(a_visited))
+
+        ## TC: O(mn), SC: O(mn)