feat: add ruby code - chapter "divide and conquer" (#1361)

codes/ruby/chapter_divide_and_conquer/build_tree.rb

@@ -0,0 +1,46 @@
+=begin
+File: build_tree.rb
+Created Time: 2024-05-13
+Author: Xuan Khoa Tu Nguyen (ngxktuzkai2000@gmail.com)
+=end
+
+require_relative '../utils/tree_node'
+require_relative '../utils/print_util'
+
+### 构建二叉树：分治 ###
+def dfs(preorder, inorder_map, i, l, r)
+  # 子树区间为空时终止
+  return if r - l < 0
+
+  # 初始化根节点
+  root = TreeNode.new(preorder[i])
+  # 查询 m ，从而划分左右子树
+  m = inorder_map[preorder[i]]
+  # 子问题：构建左子树
+  root.left = dfs(preorder, inorder_map, i + 1, l, m - 1)
+  # 子问题：构建右子树
+  root.right = dfs(preorder, inorder_map, i + 1 + m - l, m + 1, r)
+
+  # 返回根节点
+  root
+end
+
+### 构建二叉树 ###
+def build_tree(preorder, inorder)
+  # 初始化哈希表，存储 inorder 元素到索引的映射
+  inorder_map = {}
+  inorder.each_with_index { |val, i| inorder_map[val] = i }
+  dfs(preorder, inorder_map, 0, 0, inorder.length - 1)
+end
+
+### Driver Code ###
+if __FILE__ == $0
+  preorder = [3, 9, 2, 1, 7]
+  inorder = [9, 3, 1, 2, 7]
+  puts "前序遍历 = #{preorder}"
+  puts "中序遍历 = #{inorder}"
+
+  root = build_tree(preorder, inorder)
+  puts "构建的二叉树为："
+  print_tree(root)
+end