Update the codes of backtracking.

codes/csharp/chapter_backtracking/permutations_i.cs

@@ -25,6 +25,7 @@ public class permutations_i {
                 // 尝试：做出选择，更新状态
                 selected[i] = true;
                 state.Add(choice);
+                // 进行下一轮选择
                 backtrack(state, choices, selected, res);
                 // 回退：撤销选择，恢复到之前的状态
                 selected[i] = false;

codes/csharp/chapter_backtracking/permutations_ii.cs

@@ -27,6 +27,7 @@ public class permutations_ii {
                 duplicated.Add(choice); // 记录选择过的元素值
                 selected[i] = true;
                 state.Add(choice);
+                // 进行下一轮选择
                 backtrack(state, choices, selected, res);
                 // 回退：撤销选择，恢复到之前的状态
                 selected[i] = false;

codes/csharp/chapter_backtracking/preorder_traversal_iii_template.cs

@@ -49,8 +49,8 @@ public class preorder_traversal_iii_template {
             if (isValid(state, choice)) {
                 // 尝试：做出选择，更新状态
                 makeChoice(state, choice);
-                List<TreeNode> nextChoices = new List<TreeNode>() { choice.left, choice.right };
-                backtrack(state, nextChoices, res);
+                // 进行下一轮选择
+                backtrack(state, new List<TreeNode> { choice.left, choice.right }, res);
                 // 回退：撤销选择，恢复到之前的状态
                 undoChoice(state, choice);
             }