Add the initial EN translation for C++ code (#1346)

en/codes/cpp/chapter_divide_and_conquer/build_tree.cpp

@@ -0,0 +1,51 @@
+/**
+ * File: build_tree.cpp
+ * Created Time: 2023-07-17
+ * Author: krahets (krahets@163.com)
+ */
+
+#include "../utils/common.hpp"
+
+/* Build binary tree: Divide and conquer */
+TreeNode *dfs(vector<int> &preorder, unordered_map<int, int> &inorderMap, int i, int l, int r) {
+    // Terminate when subtree interval is empty
+    if (r - l < 0)
+        return NULL;
+    // Initialize root node
+    TreeNode *root = new TreeNode(preorder[i]);
+    // Query m to divide left and right subtrees
+    int m = inorderMap[preorder[i]];
+    // Subproblem: build left subtree
+    root->left = dfs(preorder, inorderMap, i + 1, l, m - 1);
+    // Subproblem: build right subtree
+    root->right = dfs(preorder, inorderMap, i + 1 + m - l, m + 1, r);
+    // Return root node
+    return root;
+}
+
+/* Build binary tree */
+TreeNode *buildTree(vector<int> &preorder, vector<int> &inorder) {
+    // Initialize hash table, storing in-order elements to indices mapping
+    unordered_map<int, int> inorderMap;
+    for (int i = 0; i < inorder.size(); i++) {
+        inorderMap[inorder[i]] = i;
+    }
+    TreeNode *root = dfs(preorder, inorderMap, 0, 0, inorder.size() - 1);
+    return root;
+}
+
+/* Driver Code */
+int main() {
+    vector<int> preorder = {3, 9, 2, 1, 7};
+    vector<int> inorder = {9, 3, 1, 2, 7};
+    cout << "Pre-order traversal = ";
+    printVector(preorder);
+    cout << "In-order traversal = ";
+    printVector(inorder);
+
+    TreeNode *root = buildTree(preorder, inorder);
+    cout << "The constructed binary tree is:\n";
+    printTree(root);
+
+    return 0;
+}