feat(csharp): add csharp code for charper divide and conquer (#628)

codes/csharp/chapter_divide_and_conquer/build_tree.cs

@@ -0,0 +1,49 @@
+/**
+* File: build_tree.cs
+* Created Time: 2023-07-18
+* Author: hpstory (hpstory1024@163.com)
+*/
+
+namespace hello_algo.chapter_divide_and_conquer;
+
+public class build_tree {
+    /* 构建二叉树：分治 */
+    public TreeNode dfs(int[] preorder, int[] inorder, Dictionary<int, int> hmap, int i, int l, int r) {
+        // 子树区间为空时终止
+        if (r - l < 0)
+            return null;
+        // 初始化根节点
+        TreeNode root = new TreeNode(preorder[i]);
+        // 查询 m ，从而划分左右子树
+        int m = hmap[preorder[i]];
+        // 子问题：构建左子树
+        root.left = dfs(preorder, inorder, hmap, i + 1, l, m - 1);
+        // 子问题：构建右子树
+        root.right = dfs(preorder, inorder, hmap, i + 1 + m - l, m + 1, r);
+        // 返回根节点
+        return root;
+    }
+
+    /* 构建二叉树 */
+    public TreeNode buildTree(int[] preorder, int[] inorder) {
+        // 初始化哈希表，存储 inorder 元素到索引的映射
+        Dictionary<int, int> hmap = new Dictionary<int, int>();
+        for (int i = 0; i < inorder.Length; i++) {
+            hmap.TryAdd(inorder[i], i);
+        }
+        TreeNode root = dfs(preorder, inorder, hmap, 0, 0, inorder.Length - 1);
+        return root;
+    }
+
+    [Test]
+    public void Test() {
+        int[] preorder = { 3, 9, 2, 1, 7 };
+        int[] inorder = { 9, 3, 1, 2, 7 };
+        Console.WriteLine("前序遍历 = " + string.Join(", ", preorder));
+        Console.WriteLine("中序遍历 = " + string.Join(", ", inorder));
+
+        TreeNode root = buildTree(preorder, inorder);
+        Console.WriteLine("构建的二叉树为：");
+        PrintUtil.PrintTree(root);
+    }
+}