solves second minimum in bst

Author: anishLearnsToCode

README.md

@@ -173,7 +173,7 @@
 | 661 | [Image Smoother](https://leetcode.com/problems/image-smoother) | [![Java](assets/java.png)](src/ImageSmoother.java) [![Python](assets/python.png)](python/image_smoother.py) |
 | 665 | [Non Deceasing Array](https://leetcode.com/problems/non-decreasing-array) | [![Java](assets/java.png)](src/NonDecreasingArray.java) [![Python](assets/python.png)](python/non_decreasing_array.py) |
 | 669 | [Trim a Binary Search Tree](https://leetcode.com/problems/trim-a-binary-search-tree) | [![Java](assets/java.png)](src/TrimABinarySearchTree.java) [![Python](assets/python.png)](python/trim_a_binary_search_tree.py) |
-| 671 | [Second Minimum Node in Binary Tree](https://leetcode.com/problems/second-minimum-node-in-a-binary-tree) | |
+| 671 | [Second Minimum Node in Binary Tree](https://leetcode.com/problems/second-minimum-node-in-a-binary-tree) | [![Java](assets/java.png)](src/SecondMinimumNodeInBinaryTree.java) [![Python](assets/python.png)](python/)second_minimum_node_in_binary_tree.py |
 | 674 | [Longest Continuous Increasing Subsequence](https://leetcode.com/problems/longest-continuous-increasing-subsequence) | |
 | 680 | [Valid Palindrome II](https://leetcode.com/problems/valid-palindrome-ii) | |
 | 682 | [Baseball Game](https://leetcode.com/problems/baseball-game) | |

python/second_minimum_node_in_binary_tree.py

@@ -0,0 +1,30 @@
+from heapq import heappop, heappush
+
+# Definition for a binary tree node.
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+
+class Solution:
+    def addNodesToSet(self, root: TreeNode, elements: set) -> None:
+        if root is None: return
+        elements.add(root.val)
+        self.addNodesToSet(root.left, elements)
+        self.addNodesToSet(root.right, elements)
+
+    def unique_elements(self, root: TreeNode) -> set:
+        elements = set()
+        self.addNodesToSet(root, elements)
+        return elements
+
+    def findSecondMinimumValue(self, root: TreeNode) -> int:
+        elements = self.unique_elements(root)
+        if len(elements) < 2: return -1
+        heap = []
+        for element in elements:
+            heappush(heap, element)
+        heappop(heap)
+        return heappop(heap)

src/SecondMinimumNodeInBinaryTree.java

@@ -0,0 +1,35 @@
+import java.util.HashSet;
+import java.util.PriorityQueue;
+import java.util.Queue;
+import java.util.Set;
+
+public class SecondMinimumNodeInBinaryTree {
+    public int findSecondMinimumValue(TreeNode root) {
+        Set<Integer> elements = treeToSet(root);
+        if (elements.size() < 2) return -1;
+        Queue<Integer> queue = setToMinHeap(elements);
+        queue.poll();
+        return queue.peek();
+    }
+
+    private static Set<Integer> treeToSet(TreeNode root) {
+        Set<Integer> result = new HashSet<>();
+        addTreeNodesToSet(root, result);
+        return result;
+    }
+
+    private static void addTreeNodesToSet(TreeNode root, Set<Integer> set) {
+        if (root == null) return;
+        set.add(root.val);
+        addTreeNodesToSet(root.left, set);
+        addTreeNodesToSet(root.right, set);
+    }
+
+    private static Queue<Integer> setToMinHeap(Set<Integer> elements) {
+        Queue<Integer> queue = new PriorityQueue<>();
+        for (int element : elements) {
+            queue.add(element);
+        }
+        return queue;
+    }
+}