[671. Second Minimum Node In a Binary Tree] #leetcode

671_0902.java

class Solution {
  // O(n) O(h)
    public int findSecondMinimumValue(TreeNode root) {
        if (root == null) return -1;
        return dfs(root, root.val);
    }
    
    private int dfs(TreeNode node, int largest) {
        if (node == null) return -1;
        if (node.val > largest) return node.val;
        int left = dfs(node.left, largest);
        int right = dfs(node.right, largest);

        if (left == -1 && right == -1) return -1;
        if (left == -1) return right;
        if (right == -1) return left;
        return Math.min(left, right);
    }
}