Add the chapter of backtracking. (#459)

codes/python/chapter_backtracking/backtrack_find_constrained_paths.py

@@ -0,0 +1,68 @@
+"""
+File: backtrack_find_constrained_path.py
+Created Time: 2023-04-15
+Author: Krahets (krahets@163.com)
+"""
+
+import sys, os.path as osp
+
+sys.path.append(osp.dirname(osp.dirname(osp.abspath(__file__))))
+from modules import *
+
+
+def is_solution(state: list[TreeNode]) -> bool:
+    """判断当前状态是否为解"""
+    return state and state[-1].val == 7
+
+
+def record_solution(state: list[TreeNode], res: list[list[TreeNode]]):
+    """记录解"""
+    res.append(list(state))
+
+
+def is_valid(state: list[TreeNode], choice: TreeNode) -> bool:
+    """判断在当前状态下，该选择是否合法"""
+    return choice is not None and choice.val != 3
+
+
+def make_choice(state: list[TreeNode], choice: TreeNode):
+    """更新状态"""
+    state.append(choice)
+
+
+def undo_choice(state: list[TreeNode], choice: TreeNode):
+    """恢复状态"""
+    state.pop()
+
+
+def backtrack(state: list[TreeNode], choices: list[TreeNode], res: list[list[TreeNode]]):
+    """回溯算法"""
+    # 检查是否为解
+    if is_solution(state):
+        # 记录解
+        record_solution(state, res)
+        return
+    # 遍历所有选择
+    for choice in choices:
+        # 剪枝：检查选择是否合法
+        if is_valid(state, choice):
+            # 尝试：做出选择，更新状态
+            make_choice(state, choice)
+            backtrack(state, [choice.left, choice.right], res)
+            # 回退：撤销选择，恢复到之前的状态
+            undo_choice(state, choice)
+
+
+"""Driver Code"""
+if __name__ == "__main__":
+    root = list_to_tree([1, 7, 3, 4, 5, 6, 7])
+    print("\n初始化二叉树")
+    print_tree(root)
+    
+    # 回溯算法
+    res = []
+    backtrack(state=[], choices=[root], res=res)
+
+    print("\n输出所有根节点到节点 7 的路径，要求路径中不包含值为 3 的节点")
+    for path in res:
+        print([node.val for node in path])