【add go for 17】

Author: 吴亲强

0-50/17.md

@@ -43,6 +43,45 @@
             return $res;
         }
 ```
+
+```go
+func letterCombinations(digits string) []string {
+	cache := make(map[string][]string)
+	cache["2"] = []string{"a", "b", "c"}
+	cache["3"] = []string{"d", "e", "f"}
+	cache["4"] = []string{"g", "h", "i"}
+	cache["5"] = []string{"j", "k", "l"}
+	cache["6"] = []string{"m", "n", "o"}
+	cache["7"] = []string{"p", "q", "r", "s"}
+	cache["8"] = []string{"t", "u", "v"}
+	cache["9"] = []string{"w", "x", "y", "z"}
+
+	var helper func(cache map[string][]string, remain string) []string
+
+	helper = func(cache map[string][]string, remain string) []string {
+		if len(remain) == 0 {
+			return []string{}
+		}
+
+		if res, ok := cache[remain]; ok {
+			return res
+		}
+
+		var res []string
+
+		first := helper(cache, remain[0:1])
+		other := helper(cache, remain[1:])
+		for _, item := range first {
+			for _, otherItem := range other {
+				res = append(res, item+otherItem)
+			}
+		}
+		return res
+	}
+	return helper(cache, digits)
+}
+```
+
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