Given a binary search tree and a range [k1, k2], return node values within a given range in ascending order.
public class Solution {
private ArrayList<Integer> results;
public ArrayList<Integer> searchRange(TreeNode root, int k1, int k2) {
results = new ArrayList<Integer>();
helper(root, k1, k2);
return results;
}
private void helper(TreeNode root, int k1, int k2) {
if (root == null) return;
if (root.val > k1) {
helper(root.left, k1, k2);
}
if (root.val >= k1 && root.val <= k2) {
results.add(root.val);
}
if (root.val < k2) {
helper(root.right, k1, k2);
}
}
}