adding bfs-1 solutions

W5_1_level_order_traversal.py

@@ -0,0 +1,42 @@
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+
+class Solution:
+    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+
+        # Self-note: Do not declare a list, pop(0) in python is O(N)
+        q = deque()
+        results = []
+
+        if root is None:
+            return []
+
+        q.append(root)
+
+        while q:
+
+            level = []
+            size = len(q)
+            for i in range(0, size):
+
+                # do not use pop(0) with a list, O(n) time in python!
+                curr = q.popleft()
+                level.append(curr.val)
+
+                if curr.left:
+                    q.append(curr.left)
+
+                if curr.right:
+                    q.append(curr.right)
+
+            results.append(level)
+
+
+        return results    
+
+# Time complexity: O(n), we visist all the nodes once.
+# Space complexity: O(n/4) = O(n), the leaf nodes (for a balance binary tree)

W5_3_course_schedule.py

@@ -0,0 +1,54 @@
+class Solution:
+    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
+
+        # Maintain a list
+        indegrees = [0] * numCourses
+
+        # Maintain a hashmap
+        outdegree_graph = {}
+
+        # We initialize the indegree by going over prerequisities and outdegrees by using a hashmap
+        for pre in prerequisites:
+            indegrees[pre[0]] = indegrees[pre[0]] + 1
+
+            if pre[1] not in outdegree_graph:
+                outdegree_graph[pre[1]] = []
+
+            outdegree_graph[pre[1]].append(pre[0])
+
+        q = deque()
+        count = 0
+
+        for i in range(0, numCourses):
+            if indegrees[i] == 0:
+                q.append(i)
+                count = count + 1
+
+        if count == numCourses:
+            return True
+
+        # if Queue is empty
+        if not q:
+            return False
+
+        while q:
+            curr = q.popleft()
+
+            # self-note: Do not use outdegree_graph.get[curr]
+            dependencies = outdegree_graph.get(curr)
+
+            if dependencies:
+                for dep in dependencies:
+                    indegrees[dep] = indegrees[dep] - 1
+                    if indegrees[dep] == 0:
+                        q.append(dep)
+                        count = count + 1
+
+                        if count == numCourses:
+                            return True 
+
+        return False
+
+# time complexity: O(v + e)
+# space complexity: O(v + e)
+