671. Second Minimum Node In a Binary Tree

本文介绍了一种算法,用于找到特殊二叉树中第二小的节点值。这种二叉树每个节点的值等于其两个子节点中的较小值。文章通过递归遍历的方法寻找最小值和第二小值,并在不存在第二小值时返回-1。

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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.


/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int findSecondMinimumValue(TreeNode root) {
        int[] data = new int[] { Integer.MAX_VALUE, Integer.MAX_VALUE };
		help(root, data);
		return data[1] != Integer.MAX_VALUE ? data[1] : -1;
    }
    
    public void help(TreeNode root, int[] data) {
		if (root == null)
			return;
		if (root.val < data[0]) {
			data[1] = data[0];
			data[0] = root.val;
		} else if (root.val < data[1] && root.val > data[0])
			data[1] = root.val;
		help(root.left, data);
		help(root.right, data);
	}
}


As an AI language model, I cannot provide you with the exact solution you need without further details. However, here is a general approach to find the longest distance in a binary tree using Python: 1. Define a class for a binary tree node with attributes for the node value, left child, and right child. 2. Define a function that recursively traverses the binary tree and returns the height of the tree starting from a given node. 3. Define a function that recursively traverses the binary tree and returns the longest distance between two nodes in the tree. 4. In the function defined in step 3, for each node, calculate the longest distance that passes through that node and update the maximum distance seen so far. 5. Call the function defined in step 3 starting from the root node of the binary tree. Here is a sample implementation of the above approach: ``` class Node: def __init__(self, value): self.value = value self.left = None self.right = None def height(root): if root is None: return 0 return 1 + max(height(root.left), height(root.right)) def longest_distance(root): if root is None: return 0 left_height = height(root.left) right_height = height(root.right) left_distance = longest_distance(root.left) right_distance = longest_distance(root.right) return max(left_height + right_height, max(left_distance, right_distance)) root = Node(1) root.left = Node(2) root.right = Node(3) root.left.left = Node(4) root.left.right = Node(5) root.right.left = Node(6) root.right.right = Node(7) print(longest_distance(root)) ``` This code will output the longest distance between any two nodes in the binary tree.
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