Update bfs and dfs(iterative) python code.

Author: zhuj05

graph/bfs.py

@@ -0,0 +1,39 @@
+from collections import deque
+
+def bfs(graph, start):
+    # 初始化已訪問集合 (Visited Set)，避免重複遍歷節點導致無窮迴圈
+    visited = {start}
+    
+    # 初始化雙端佇列 (Deque)，並將起點放入佇列中作為 BFS 的起始點
+    queue = deque([start])
+
+    # 當佇列中還有節點時，繼續搜尋
+    while queue:
+        # 從佇列左側取出最早進入的節點 (Popleft)，實現 FIFO (First-In-First-Out)
+        node = queue.popleft()
+        
+        # 輸出目前的節點
+        print(node, end=" ")
+        
+        # 遍歷目前節點的所有鄰居 (Neighbors)
+        for neighbor in graph[node]:
+            # 如果該鄰居節點尚未被訪問過
+            if neighbor not in visited:
+                # 將其標記為已訪問 (Visited)
+                visited.add(neighbor)
+                # 將其加入佇列末端 (Append)，排隊等待下一層搜尋
+                queue.append(neighbor)
+
+# 定義鄰接表 (Adjacency List)，表示圖的結構
+graph = {
+    'A': ['B','C'],
+    'B': ['A','D','E'],
+    'C': ['A','F'],
+    'D': ['B'],
+    'E': ['B','F'],
+    'F': ['C','E']
+}
+
+# 從節點 'A' 開始執行廣度優先搜尋
+bfs(graph, 'A')
+# 預期輸出: A B C D E F

graph/dfs_iterative.py

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+# Depth-First Search (DFS) - Iterative Approach(深度優先搜尋-迭代法)
+def dfs_iterative(graph, start):
+    # Initialize an empty set to keep track of visited nodes to avoid infinite loops
+    visited = set()
+    
+    # Initialize a stack with the starting node
+    stack = [start]
+    
+    # Continue searching as long as there are nodes in the stack
+    while stack:
+        # Remove and return the last element added to the stack (LIFO)
+        node = stack.pop()
+        
+        # Check if the node has been visited before
+        if node not in visited:
+            # Print the current node (or perform other operations)
+            print(node, end=" ")
+            
+            # Mark the current node as visited
+            visited.add(node)
+            
+            # Add all unvisited neighbors of the current node to the stack
+            # Note: The order in the stack determines the next node to visit
+            stack.extend(graph[node])
+
+# Define the graph using an adjacency list
+graph = {
+    'A': ['B', 'C'],
+    'B': ['A', 'D', 'E'],
+    'C': ['A', 'F'],
+    'D': ['B'],
+    'E': ['B', 'F'],
+    'F': ['C', 'E']
+}
+
+# Execute the DFS starting from node 'A'
+# Output: A C F E B D 
+dfs_iterative(graph, 'A')