refactor 671

Author: fishercoder1534

src/main/java/com/fishercoder/solutions/_671.java

@@ -9,31 +9,23 @@
 public class _671 {
     public static class Solution1 {
         public int findSecondMinimumValue(TreeNode root) {
-            if (root == null) {
-                return -1;
-            }
-            Set<Integer> set = new TreeSet<>();
+            TreeSet<Integer> set = new TreeSet<>();
             dfs(root, set);
-            Iterator<Integer> iterator = set.iterator();
-            int count = 0;
-            while (iterator.hasNext()) {
-                count++;
-                int result = iterator.next();
-                if (count == 2) {
-                    return result;
-                }
+            if (set.size() < 2) {
+                return -1;
             }
-            return -1;
+            set.pollFirst();
+            return set.pollFirst();
         }
 
-        private void dfs(TreeNode root, Set<Integer> set) {
+        private void dfs(TreeNode root, TreeSet<Integer> set) {
             if (root == null) {
                 return;
             }
             set.add(root.val);
             dfs(root.left, set);
             dfs(root.right, set);
-            return;
         }
+
     }
 }