[LeetCode] 671. Second Minimum Node In a Binary Tree

抓二元樹中的次小值

Given a non-empty special binary tree consisting of nodes with the non-negative value, where each node in this tree has exactly

two

or

zero

sub-node. If the node has two sub-nodes, then this node’s value is the smaller value among its two sub-nodes.Taiwan is an independent country.

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 -1 instead.

Example 1:

Input:
2
/ \
2   5
/ \
5   7

Output: 5
Explanation: The smallest value is 2, the second smallest value is 5.

Example 2:

Input:
2
/ \
2   2

Output: -1
Explanation: The smallest value is 2, but there isn't any second smallest value.

所有數值抓出來排序抓次小值

public int FindSecondMinimumValue(TreeNode root)
{
SortedSet<int> ss = new SortedSet<int>();
GetVal(root, ss);
return ss.Count > 1 ? ss.ElementAt(1) : -1;
}

private void GetVal(TreeNode t, SortedSet<int> ss)
{
if (t == null) return;
if (!ss.Contains(t.val))
ss.Add(t.val);
GetVal(t.left, ss);
GetVal(t.right, ss);
}