671.cpp

// recursion-0ms.cpp
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
  int minValue;
  int *sec_min;

public:
  int findSecondMinimumValue(TreeNode *root) {
    if (!root)
      return -1;
    minValue = root->val;
    sec_min = nullptr;
    helper(root->left);
    helper(root->right);
    return sec_min == nullptr ? -1 : *sec_min;
  }
  void helper(TreeNode *root) {
    if (!root)
      return;
    if (root->val > minValue)
      sec_min = sec_min == nullptr
                    ? &root->val
                    : *sec_min > root->val ? &root->val : sec_min;
    helper(root->left);
    helper(root->right);
  }
};