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// @Title: 二叉树中第二小的节点 (Second Minimum Node In a Binary Tree)
// @Author: 15816537946@163.com
// @Date: 2019-11-25 10:23:06
// @Runtime: 0 ms
// @Memory: 1.9 MB
/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */

 func findSecondMinimumValue(root *TreeNode) int {
	max := 1<<63 - 1
	hi := max
	var helper func(*TreeNode, int)
	helper = func(node *TreeNode, lo int) {
		if node == nil {
			return
		}
		if lo < node.Val && node.Val < hi {
			hi = node.Val
		}
		helper(node.Left, lo)
		helper(node.Right, lo)
	}

	helper(root, root.Val)

	if max == hi {
		return -1
	}

	return hi
}