add 671

Author: fishercoder1534

README.md

@@ -22,6 +22,7 @@ Your ideas/fixes/algorithms are more than welcome!
 
 |  #  |      Title     |   Solutions   | Time          | Space         | Difficulty  | Tag          | Notes
 |-----|----------------|---------------|---------------|---------------|-------------|--------------|-----
+|671|[Second Minimum Node In a Binary Tree](https://leetcode.com/problems/second-minimum-node-in-a-binary-tree/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_671.java) | O(n) | O(n) | Easy | Tree, DFS
 |668|[Kth Smallest Number in Multiplication Table](https://leetcode.com/problems/kth-smallest-number-in-multiplication-table/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_668.java) | O(logm*n) | O(1) | Hard | Binary Search
 |667|[Beautiful Arrangement II](https://leetcode.com/problems/beautiful-arrangement-ii/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_667.java) | O(n) | O(1) | Medium | Array
 |666|[Path Sum IV](https://leetcode.com/problems/path-sum-iv/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_666.java) | O(1) | O(1) | Medium | Tree, DFS

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

@@ -0,0 +1,71 @@
+package com.fishercoder.solutions;
+
+import com.fishercoder.common.classes.TreeNode;
+
+import java.util.Iterator;
+import java.util.Set;
+import java.util.TreeSet;
+
+/**
+ * 671. Second Minimum Node In a Binary Tree
+ *
+ *  Given a non-empty special binary tree consisting of nodes with the non-negative value,
+ *  where each node in this tree has exactly two or zero sub-node.
+ *  If the node has two sub-nodes, then this node's value is the smaller value among its two sub-nodes.
+ *  Given such a binary tree, you need to output the second minimum value in the set made of all the nodes' value in the whole tree.
+ *  If no such second minimum value exists, output -1 instead.
+
+ Example 1:
+
+ Input:
+   2
+  / \
+ 2   5
+    / \
+   5   7
+
+ Output: 5
+ Explanation: The smallest value is 2, the second smallest value is 5.
+
+ Example 2:
+
+ Input:
+  2
+ / \
+2  2
+
+ Output: -1
+ Explanation: The smallest value is 2, but there isn't any second smallest value.
+
+ */
+public class _671 {
+    public static class Solution1 {
+        public int findSecondMinimumValue(TreeNode root) {
+            if (root == null) {
+                return -1;
+            }
+            Set<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;
+                }
+            }
+            return -1;
+        }
+
+        private void dfs(TreeNode root, Set<Integer> set) {
+            if (root == null) {
+                return;
+            }
+            set.add(root.val);
+            dfs(root.left, set);
+            dfs(root.right, set);
+            return;
+        }
+    }
+}

src/test/java/com/fishercoder/_671Test.java

@@ -0,0 +1,34 @@
+package com.fishercoder;
+
+import com.fishercoder.common.classes.TreeNode;
+import com.fishercoder.common.utils.TreeUtils;
+import com.fishercoder.solutions._671;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import java.util.Arrays;
+
+import static org.junit.Assert.assertEquals;
+
+public class _671Test {
+    private static _671.Solution1 solution1;
+    private static TreeNode root;
+
+    @BeforeClass
+    public static void setup() {
+        solution1 = new _671.Solution1();
+    }
+
+    @Test
+    public void test1() {
+        root = TreeUtils.constructBinaryTree(Arrays.asList(2, 2, 5, null, null, 5, 7));
+        assertEquals(5, solution1.findSecondMinimumValue(root));
+    }
+
+    @Test
+    public void test2() {
+        root = TreeUtils.constructBinaryTree(Arrays.asList(2, 2, 2));
+        assertEquals(-1, solution1.findSecondMinimumValue(root));
+    }
+
+}