[Leo] 9th Week solutions

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))

design-add-and-search-words-data-structure/Leo.py

@@ -0,0 +1,48 @@
+class TrieNode:
+    def __init__(self):
+        self.links = {}
+        self.end = False
+
+
+class WordDictionary:
+    def __init__(self):
+        self.root = TrieNode()
+        self.maxL = 0
+
+    def addWord(self, word: str) -> None:
+        node = self.root
+        l = 0
+
+        for w in word:
+            if w not in node.links:
+                node.links[w] = TrieNode()
+            node = node.links[w]
+            l += 1
+
+        self.maxL = max(self.maxL, l)
+        node.end = True
+
+        ## TC: O(len(word)), SC: O(1)
+
+    def search(self, word: str) -> bool:
+        if len(word) > self.maxL:
+            return False
+
+        def helper(index, node):
+            for inn in range(index, len(word)):
+                c = word[inn]
+                if c == ".":
+                    for child in node.links.values():
+                        if helper(inn + 1, child):
+                            return True
+                    return False
+                else:
+                    if c not in node.links:
+                        return False
+                    node = node.links[c]
+
+            return node.end
+
+        return helper(0, self.root)
+
+        ## TC: O(num(node)), SC: O(len(word))

number-of-islands/Leo.py

@@ -0,0 +1,25 @@
+class Solution:
+    def numIslands(self, grid: List[List[str]]) -> int:
+        res = 0
+        rows, cols, = len(grid), len(grid[0])
+
+        def dfs(x, y):
+            if x < 0 or y < 0 or x >= rows or y >= cols or grid[x][y] == "0":
+                return
+
+            if grid[x][y] == "1":
+                grid[x][y] = "0"
+                dfs(x + 1, y)
+                dfs(x - 1, y)
+                dfs(x, y + 1)
+                dfs(x, y - 1)
+
+        for i in range(rows):
+            for j in range(cols):
+                if grid[i][j] == "1":
+                    dfs(i, j)
+                    res += 1
+
+        return res
+
+        ## TC & SC: O(m*n)

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)