添加 0020.有效的括号 Ruby 版本

problems/0015.三数之和.md

@@ -336,6 +336,23 @@ var threeSum = function(nums) {
 ```
 
 
+ruby:
+```ruby
+def is_valid(strs)
+    symbol_map = {')' => '(', '}' => '{', ']' => '['}
+    stack = []
+    strs.size.times {|i|
+        c = strs[i]
+        if symbol_map.has_key?(c)
+            top_e = stack.shift
+            return false if symbol_map[c] != top_e
+        else
+            stack.unshift(c)
+        end
+    }
+    stack.empty?
+end
+```
 
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