671. Second Minimum Node In a Binary Tree

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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);
	}
}