Update the codes of backtracking.

codes/java/chapter_backtracking/permutations_i.java

@@ -24,6 +24,7 @@ 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/java/chapter_backtracking/permutations_ii.java

@@ -26,6 +26,7 @@ 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/java/chapter_backtracking/preorder_traversal_iii_template.java

@@ -49,6 +49,7 @@ public class preorder_traversal_iii_template {
             if (isValid(state, choice)) {
                 // 尝试：做出选择，更新状态
                 makeChoice(state, choice);
+                // 进行下一轮选择
                 backtrack(state, Arrays.asList(choice.left, choice.right), res);
                 // 回退：撤销选择，恢复到之前的状态
                 undoChoice(state, choice);