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671. Second Minimum Node In a Binary Tree

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Description

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. More formally, the property root.val = min(root.left.val, root.right.val) always holds.

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: root = [2,2,5,null,null,5,7]
Output: 5
Explanation: The smallest value is 2, the second smallest value is 5.

Example 2:

Input: root = [2,2,2]
Output: -1
Explanation: The smallest value is 2, but there isn't any second smallest value.

 

Constraints:

• The number of nodes in the tree is in the range [1, 25].
• 1 <= Node.val <= 231 - 1
• root.val == min(root.left.val, root.right.val) for each internal node of the tree.

Solutions

Python3

# 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 findSecondMinimumValue(self, root: TreeNode) -> int:
        def traverse(root, val):
            if root is None:
                return
            traverse(root.left, val)
            traverse(root.right, val)
            if root.val > val:
                self.res = root.val if self.res == -1 else min(self.res, root.val)

        self.res = -1
        traverse(root, root.val)
        return self.res

Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    private int res;
    public int findSecondMinimumValue(TreeNode root) {
        res = -1;
        traverse(root, root.val);
        return res;
    }

    private void traverse(TreeNode root, int min) {
        if (root == null) {
            return;
        }
        traverse(root.left, min);
        traverse(root.right, min);
        if (root.val > min) {
            res = res == -1 ? root.val : Math.min(res, root.val);
        }
    }
}

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