refactor(lcof): simplify string joining in problem 45 Java solution

Author: yanglbme

lcof/面试题45. 把数组排成最小的数/README.md

@@ -77,8 +77,7 @@ class Solution {
         return Arrays.stream(nums)
             .mapToObj(String::valueOf)
             .sorted((a, b) -> (a + b).compareTo(b + a))
-            .reduce((a, b) -> a + b)
-            .orElse("");
+            .collect(Collectors.joining());
     }
 }
 ```

lcof/面试题45. 把数组排成最小的数/Solution.java

@@ -3,7 +3,6 @@ public String minNumber(int[] nums) {
         return Arrays.stream(nums)
             .mapToObj(String::valueOf)
             .sorted((a, b) -> (a + b).compareTo(b + a))
-            .reduce((a, b) -> a + b)
-            .orElse("");
+            .collect(Collectors.joining());
     }
-}
+}