LeetCode 701 Insert into a Binary Search Tree

给予树：
        4 
       / \ 
      2   7 
     / \ 
    1   3 
并且要插入的值：5

     4
   /   \
  2     7
 / \   /
1   3 5

     5
   /   \
  2     7
 / \   
1   3
     \
      4

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode insertIntoBST(TreeNode root, int val) {
        if (root == null) {
            throw new IllegalArgumentException("Tree Root can not be empty");
        }
        TreeNode current = root;

        TreeNode preNode = null;
        while (current != null) {
            preNode = current;
            if (current.val < val) {
                current = current.right;
            } else if (current.val > val){
                current = current.left;
            }
        }

        if (preNode.val < val) {
            System.out.println("1");
            preNode.right = new TreeNode(val);
        } else {
            System.out.println("2");
            preNode.left = new TreeNode(val);
        }

        return root;
    }
}