Click here to see the full article post
@xwEm [null] input is invalid and  will never be the input as binary tree is non-empty.
But for other problems, such as 606. The official solution will first check if the input is nullptr. https://leetcode.com/articles/construct-string-from-binary-tree/
Looks like your connection to LeetCode Discuss was lost, please wait while we try to reconnect.