Closest Binary Search Tree Value

Given a non-empty binary search tree and a target value, find the value in the BST that is closest to the target.
Note:

• Given target value is a floating point.
• You are guaranteed to have only one unique value in the BST that is closest to the target.
• 
  

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {

    public int closestValue(TreeNode root, double target) {
        Stack<TreeNode> stk = new Stack<TreeNode>();
        TreeNode cur = root;
        TreeNode pre = null;
     
        while (!stk.isEmpty() || cur != null) {
            if (cur != null) {
                stk.push(cur);
                cur = cur.left;
            } else {
                TreeNode node = stk.pop();
                // visit
                if (target < node.val) {
                    if (pre == null) return node.val;
                    else {
                        return Math.abs(node.val - target) > Math.abs(pre.val - target) ? pre.val : node.val;
                    }
                }
                pre = node;
                cur = node.right;
            }
        }
        return pre.val;
    }
}

================

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int closestValue(TreeNode root, double target) {
        TreeNode cur = root;
        TreeNode res = root;
        while (cur != null) {
            if (Math.abs(cur.val - target) < Math.abs(res.val - target)) {
                res = cur;
            }
            if (cur.val > target) {
                cur = cur.left;
            } else {
                cur = cur.right;
            }
        }
        return res.val;
    }
}