- The root is the min val in the tree.
- Set up
secondMin = Infinity.
- Traverse all nodes, and compare with
val < secondMin && val > Min.
Given a non-empty special binary tree consisting of nodes with the non-negative value, where each node in this tree has exactly
zero sub-node. If the node has two sub-nodes, then this node’s value is the smaller value among its two sub-nodes. More formally, the property
root.val = min(root.left.val, root.right.val) always holds.
Given such a binary tree, you need to output the
second minimum value in the set made of all the nodes’ value in the whole tree.
If no such second minimum value exists, output
Input: root = [2,2,5,null,null,5,7]