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class Solution {
    public int closestValue(TreeNode root, double target) {
        int res = root.val;
        while (root != null) {
            if (Math.abs(root.val - target) < Math.abs(res - target)) {
                res = root.val;
            }
            root = root.val > target ? root.left : root.right;
        }
        return res;
    }
}