采用递归的方式中根遍历

先从小到大排序放进list中，再根据索引get

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param proot TreeNode类 
     * @param k int整型 
     * @return int整型
     */
    public int KthNode (TreeNode proot, int k) {
        // write code here
        if(proot==null) return -1;
//         先找到最小值
        LinkedList<TreeNode> linkedlist = new LinkedList<>();
        LinkedList<Integer> list =inorder(proot,new LinkedList<>());
        return k>0&&k<=list.size()?list.get(k-1):-1;
        
    }
    private LinkedList<Integer> inorder(TreeNode treenode,LinkedList<Integer> linkedlist){
        if(treenode!=null){
            inorder(treenode.left,linkedlist);
            linkedlist.add(treenode.val);
            inorder(treenode.right,linkedlist);
            return linkedlist;
        }
        return linkedlist;
    }
}