Solution2.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode trimBST(TreeNode root, int low, int high) {
        while (root != null && (root.val < low || root.val > high)) {
            root = root.val < low ? root.right : root.left;
        }
        if (root == null) {
            return null;
        }
        TreeNode node = root;
        while (node.left != null) {
            if (node.left.val < low) {
                node.left = node.left.right;
            } else {
                node = node.left;
            }
        }
        node = root;
        while (node.right != null) {
            if (node.right.val > high) {
                node.right = node.right.left;
            } else {
                node = node.right;
            }
        }
        return root;
    }
}