Solution.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 ans = -1;

    public int findSecondMinimumValue(TreeNode root) {
        dfs(root, root.val);
        return ans;
    }

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