【easy】671. Second Minimum Node In a Binary Tree

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.

题目：

特殊的二叉树，父节点是子节点中较小者，找出二叉树中次小的值。

********这个题用java写的…java中是null, 并且数组是‘引用’，不用 - > 用 .

Input: 
    2
   / 
  2   5
     / 
    5   7

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

Input: 
    2
   / 
  2   2

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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int findSecondMinimumValue(TreeNode root) {
        int []data = new int []{Integer.MAX_VALUE, Integer.MAX_VALUE};
        help(root,data);
        return data[1]!=Integer.MAX_VALUE?data[1]:-1;
    }
    
    public void help(TreeNode root, int []data){//java直接就是引用
        if (root == null)
            return;
        if (root.val<data[0]){
            data[1] = data[0];
            data[0] = root.val;
        }
        if (root.val<data[1] && root.val>data[0])
            data[1] = root.val;
        
        help(root.left,data);
        help(root.right,data);
    }
}