解题思路

首先进行前序遍历，将二叉树所有的节点存入set中，然后再放入res中。

相关代码

/*** 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 {Set<Integer> set = new HashSet<>();public int findSecondMinimumValue(TreeNode root) {dfs(root);if(set.size()<2) return -1;List<Integer> res = new ArrayList<>();for(Integer temp:set){res.add(temp);}Collections.sort(res);return res.get(1);}public void dfs(TreeNode root){if(root==null) return;set.add(root.val);dfs(root.left);dfs(root.right);}
}