递归。满足条件跳出递归

class Solution {
    public TreeNode kthNode(TreeNode root, int k) {
        Solution s = new Solution();
        List<TreeNode> list = new ArrayList();
        return s.zx(root,k,list).get(k-1);
    }

    public List<TreeNode> zx(TreeNode root,int k,List<TreeNode> list){
        if(root.left != null){
            zx(root.left,k,list);
        }
        if(root != null){
            list.add(root);
            if(list.size() == k) {
                new RuntimeException();
            }
        }
        if(root.right != null){
            zx(root.right,k,list);
        }
        return list;
    }

}

非递归中序

class Solution {
    public TreeNode kthNode(TreeNode root, int k) {
        Stack<TreeNode> stack = new Stack();
        stack.push(root);
        int count = 0;
        while(root!=null || !stack.isEmpty()){
            while(root!=null){
                stack.push(root);
                root = root.left;
            }
            if(!stack.isEmpty()){
                TreeNode t = stack.pop();//弹出根节点
                count++;
                if(count == k){
                    return t;
                }
                root = t.right;
            }

        }
        return root;
    }
}