Add cpp code for the backtrack algorithm.

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);
+}