Merge pull request #2255 from niuli1991/master-77

添加0077.组合 Ruby实现

problems/0077.组合.md

@@ -764,6 +764,35 @@ object Solution {
 }
 ```
 
+### Ruby
+
+```ruby
+
+def combine(n, k)
+  result = []
+  path = []
+  backtracking(result, path, n, 1, k)
+  return result
+end
+
+#剪枝优化
+def backtracking(result, path, n, j, k)
+  if path.size == k
+    result << path.map {|item| item}
+    return
+  end
+
+  for i in j..(n-(k - path.size)) + 1
+    #处理节点
+    path << i
+    backtracking(result, path, n, i + 1, k)
+    #回溯，撤销处理过的节点
+    path.pop
+  end
+end
+
+```
+
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