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

Problem

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.

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.

Solution

class Solution {
    public int findSecondMinimumValue(TreeNode root) {
        if(root == null || root.left == null) return -1;
        
        int leftRes, rightRes;
        if (root.left.val == root.val) {
            leftRes = findSecondMinimumValue(root.left);
        } else {
            leftRes = root.left.val;
        }
        if (root.right.val == root.val) {
            rightRes = findSecondMinimumValue(root.right);
        } else {
            rightRes = root.right.val;
        }
        
        if (leftRes == -1 && rightRes == -1) return -1;
        else if (leftRes == -1) return rightRes;
        else if (rightRes == -1) return leftRes;
        else return Math.min(leftRes, rightRes);

    }
}