_671.java

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;
        }
    }
}