Merge pull request #2265 from niuli1991/master-17

添加0017.电话号码的字母组合 Ruby实现

problems/0017.电话号码的字母组合.md

@@ -694,6 +694,44 @@ object Solution {
 }
 ```
 
+### Ruby
+```ruby
+def letter_combinations(digits)
+  letter_map = {
+    2 => ['a','b','c'],
+    3 => ['d','e','f'],
+    4 => ['g','h','i'],
+    5 => ['j','k','l'],
+    6 => ['m','n','o'],
+    7 => ['p','q','r','s'],
+    8 => ['t','u','v'],
+    9 => ['w','x','y','z']
+  }
+  
+  result = []
+  path = []
+
+  return result if digits.size == 0
+
+  backtracking(result, letter_map, digits.split(''), path, 0)
+  result
+end
+
+def backtracking(result, letter_map, digits, path, index) 
+  if path.size == digits.size
+    result << path.join('')
+    return
+  end
+
+  hash[digits[index].to_i].each do |chr|
+    path << chr
+    #index + 1代表处理下一个数字
+    backtracking(result, letter_map, digits, path, index + 1)
+    #回溯，撤销处理过的数字
+    path.pop
+  end
+end
+```
 
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