Add cpp code for the backtrack algorithm.

codes/cpp/chapter_backtracking/backtrack_find_constrained_paths.cpp

@@ -0,0 +1,75 @@
+/**
+ * File: backtrack_find_constrained_paths.cpp
+ * Created Time: 2023-04-16
+ * Author: Krahets (krahets@163.com)
+ */
+
+#include "../include/include.hpp"
+
+/* 判断当前状态是否为解 */
+bool isSolution(vector<TreeNode *> &state) {
+    return !state.empty() && state.back()->val == 7;
+}
+
+/* 记录解 */
+void recordSolution(vector<TreeNode *> &state, vector<vector<TreeNode *>> &res) {
+    res.push_back(state);
+}
+
+/* 判断在当前状态下，该选择是否合法 */
+bool isValid(vector<TreeNode *> &state, TreeNode *choice) {
+    return choice != nullptr && choice->val != 3;
+}
+
+/* 更新状态 */
+void makeChoice(vector<TreeNode *> &state, TreeNode *choice) {
+    state.push_back(choice);
+}
+
+/* 恢复状态 */
+void undoChoice(vector<TreeNode *> &state, TreeNode *choice) {
+    state.pop_back();
+}
+
+/* 回溯算法 */
+void backtrack(vector<TreeNode *> &state, vector<TreeNode *> &choices, vector<vector<TreeNode *>> &res) {
+    // 检查是否为解
+    if (isSolution(state)) {
+        // 记录解
+        recordSolution(state, res);
+        return;
+    }
+    // 遍历所有选择
+    for (TreeNode *choice : choices) {
+        // 剪枝：检查选择是否合法
+        if (isValid(state, choice)) {
+            // 尝试：做出选择，更新状态
+            makeChoice(state, choice);
+            vector<TreeNode *> nextChoices{choice->left, choice->right};
+            backtrack(state, nextChoices, res);
+            // 回退：撤销选择，恢复到之前的状态
+            undoChoice(state, choice);
+        }
+    }
+}
+
+int main() {
+    TreeNode *root = vecToTree(vector<int>{1, 7, 3, 4, 5, 6, 7});
+    cout << "\n初始化二叉树" << endl;
+    printTree(root);
+
+    // 回溯算法
+    vector<TreeNode *> state;
+    vector<TreeNode *> choices = {root};
+    vector<vector<TreeNode *>> res;
+    backtrack(state, choices, res);
+
+    cout << "\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" << endl;
+    for (vector<TreeNode *> &path : res) {
+        vector<int> vals;
+        for (TreeNode *node : path) {
+            vals.push_back(node->val);
+        }
+        printVector(vals);
+    }
+}

codes/cpp/chapter_backtracking/preorder_find_constrained_paths.cpp

@@ -0,0 +1,45 @@
+/**
+ * File: preorder_find_constrained_paths.cpp
+ * Created Time: 2023-04-16
+ * Author: Krahets (krahets@163.com)
+ */
+
+#include "../include/include.hpp"
+
+vector<TreeNode *> path;
+vector<vector<TreeNode *>> res;
+
+/* 前序遍历 */
+static void preOrder(TreeNode *root) {
+    if (root == nullptr || root->val == 3) {
+        return;
+    }
+    // 尝试
+    path.push_back(root);
+    if (root->val == 7) {
+        // 记录解
+        res.push_back(path);
+    }
+    preOrder(root->left);
+    preOrder(root->right);
+    // 回退
+    path.pop_back();
+}
+
+int main() {
+    TreeNode *root = vecToTree(vector<int>{1, 7, 3, 4, 5, 6, 7});
+    cout << "\n初始化二叉树" << endl;
+    printTree(root);
+
+    // 前序遍历
+    preOrder(root);
+
+    cout << "\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" << endl;
+    for (vector<TreeNode *> &path : res) {
+        vector<int> vals;
+        for (TreeNode *node : path) {
+            vals.push_back(node->val);
+        }
+        printVector(vals);
+    }
+}

codes/cpp/chapter_backtracking/preorder_find_nodes.cpp

@@ -0,0 +1,38 @@
+/**
+ * File: preorder_find_nodes.cpp
+ * Created Time: 2023-04-16
+ * Author: Krahets (krahets@163.com)
+ */
+
+#include "../include/include.hpp"
+
+vector<TreeNode *> res;
+
+/* 前序遍历 */
+static void preOrder(TreeNode *root) {
+    if (root == nullptr) {
+        return;
+    }
+    if (root->val == 7) {
+        // 记录解
+        res.push_back(root);
+    }
+    preOrder(root->left);
+    preOrder(root->right);
+}
+
+int main() {
+    TreeNode *root = vecToTree(vector<int>{1, 7, 3, 4, 5, 6, 7});
+    cout << "\n初始化二叉树" << endl;
+    printTree(root);
+
+    // 前序遍历
+    preOrder(root);
+
+    cout << "\n输出所有值为 7 的节点" << endl;
+    vector<int> vals;
+    for (TreeNode *node : res) {
+        vals.push_back(node->val);
+    }
+    printVector(vals);
+}

codes/cpp/chapter_backtracking/preorder_find_paths.cpp

@@ -0,0 +1,45 @@
+/**
+ * File: preorder_find_paths.cpp
+ * Created Time: 2023-04-16
+ * Author: Krahets (krahets@163.com)
+ */
+
+#include "../include/include.hpp"
+
+vector<TreeNode *> path;
+vector<vector<TreeNode *>> res;
+
+/* 前序遍历 */
+static void preOrder(TreeNode *root) {
+    if (root == nullptr) {
+        return;
+    }
+    // 尝试
+    path.push_back(root);
+    if (root->val == 7) {
+        // 记录解
+        res.push_back(path);
+    }
+    preOrder(root->left);
+    preOrder(root->right);
+    // 回退
+    path.pop_back();
+}
+
+int main() {
+    TreeNode *root = vecToTree(vector<int>{1, 7, 3, 4, 5, 6, 7});
+    cout << "\n初始化二叉树" << endl;
+    printTree(root);
+
+    // 前序遍历
+    preOrder(root);
+
+    cout << "\n输出所有根节点到节点 7 的路径" << endl;
+    for (vector<TreeNode *> &path : res) {
+        vector<int> vals;
+        for (TreeNode *node : path) {
+            vals.push_back(node->val);
+        }
+        printVector(vals);
+    }
+}

codes/java/chapter_backtracking/preorder_find_nodes.java

@@ -32,7 +32,7 @@ public class preorder_find_nodes {
         res = new ArrayList<>();
         preOrder(root);
 
-        System.out.println("\n输出所有根节点到节点 7 的路径");
+        System.out.println("\n输出所有值为 7 的节点");
         List<Integer> vals = new ArrayList<>();
         for (TreeNode node : res) {
             vals.add(node.val);