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

codes/ruby/chapter_divide_and_conquer/binary_search_recur.rb

@@ -0,0 +1,42 @@
+=begin
+File: binary_search_recur.rb
+Created Time: 2024-05-13
+Author: Xuan Khoa Tu Nguyen (ngxktuzkai2000@gmail.com)
+=end
+
+### 二分查找：问题 f(i, j) ###
+def dfs(nums, target, i, j)
+  # 若区间为空，代表无目标元素，则返回 -1
+  return -1 if i > j
+  
+  # 计算中点索引 m
+  m = (i + j) / 2
+
+  if nums[m] < target
+    # 递归子问题 f(m+1, j)
+    return dfs(nums, target, m + 1, j)
+  elsif nums[m] > target
+    # 递归子问题 f(i, m-1)
+    return dfs(nums, target, i, m - 1)
+  else
+    # 找到目标元素，返回其索引
+    return m
+  end
+end
+
+### 二分查找 ###
+def binary_search(nums, target)
+  n = nums.length
+  # 求解问题 f(0, n-1)
+  dfs(nums, target, 0, n - 1)
+end
+
+### Driver Code ###
+if __FILE__ == $0
+  target = 6
+  nums = [1, 3, 6, 8, 12, 15, 23, 26, 31, 35]
+
+  # 二分查找（双闭区间）
+  index = binary_search(nums, target)
+  puts "目标元素 6 的索引 = #{index}"
+end