Fix panic template

codes/rust/chapter_backtracking/preorder_traversal_iii_template.rs

@@ -11,7 +11,7 @@ use tree_node::{vec_to_tree, TreeNode};
 
 /* 判断当前状态是否为解 */
 fn is_solution(state: &mut Vec<Rc<RefCell<TreeNode>>>) -> bool {
-    return !state.is_empty() && state.get(state.len() - 1).unwrap().borrow().val == 7;
+    return !state.is_empty() && state.last().unwrap().borrow().val == 7;
 }
 
 /* 记录解 */
@@ -23,8 +23,8 @@ fn record_solution(
 }
 
 /* 判断在当前状态下，该选择是否合法 */
-fn is_valid(_: &mut Vec<Rc<RefCell<TreeNode>>>, choice: Rc<RefCell<TreeNode>>) -> bool {
-    return choice.borrow().val != 3;
+fn is_valid(_: &mut Vec<Rc<RefCell<TreeNode>>>, choice: Option<&Rc<RefCell<TreeNode>>>) -> bool {
+    return choice.is_some() && choice.unwrap().borrow().val != 3;
 }
 
 /* 更新状态 */
@@ -34,13 +34,13 @@ fn make_choice(state: &mut Vec<Rc<RefCell<TreeNode>>>, choice: Rc<RefCell<TreeNo
 
 /* 恢复状态 */
 fn undo_choice(state: &mut Vec<Rc<RefCell<TreeNode>>>, _: Rc<RefCell<TreeNode>>) {
-    state.remove(state.len() - 1);
+    state.pop();
 }
 
 /* 回溯算法：例题三 */
 fn backtrack(
     state: &mut Vec<Rc<RefCell<TreeNode>>>,
-    choices: &mut Vec<Rc<RefCell<TreeNode>>>,
+    choices: &Vec<Option<&Rc<RefCell<TreeNode>>>>,
     res: &mut Vec<Vec<Rc<RefCell<TreeNode>>>>,
 ) {
     // 检查是否为解
@@ -49,22 +49,22 @@ fn backtrack(
         record_solution(state, res);
     }
     // 遍历所有选择
-    for choice in choices {
+    for &choice in choices.iter() {
         // 剪枝：检查选择是否合法
-        if is_valid(state, choice.clone()) {
+        if is_valid(state, choice) {
             // 尝试：做出选择，更新状态
-            make_choice(state, choice.clone());
+            make_choice(state, choice.unwrap().clone());
             // 进行下一轮选择
             backtrack(
                 state,
-                &mut vec![
-                    choice.borrow().left.clone().unwrap(),
-                    choice.borrow().right.clone().unwrap(),
+                &vec![
+                    choice.unwrap().borrow().left.as_ref(),
+                    choice.unwrap().borrow().right.as_ref(),
                 ],
                 res,
             );
             // 回退：撤销选择，恢复到之前的状态
-            undo_choice(state, choice.clone());
+            undo_choice(state, choice.unwrap().clone());
         }
     }
 }
@@ -77,7 +77,7 @@ pub fn main() {
 
     // 回溯算法
     let mut res = Vec::new();
-    backtrack(&mut Vec::new(), &mut vec![root.unwrap()], &mut res);
+    backtrack(&mut Vec::new(), &mut vec![root.as_ref()], &mut res);
 
     println!("\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 {