Lintcode: Search Range in Binary Search Tree

Given two values k1 and k2 (where k1 < k2) and a root pointer to a Binary Search Tree. Find all the keys of tree in range k1 to k2. i.e. print all x such that k1<=x<=k2 and x is a key of given BST. Return all the keys in ascending order.

Example
For example, if k1 = 10 and k2 = 22, then your function should print 12, 20 and 22.

          20

       /        \

    8           22

  /     \

4       12

我的做法是inorder traversal的变形，判断是否向左边递归的时候加上判断是否：root.val > k1, 如果否，则不需要继续向左递归；右子树的处理方法类似

第一次做法，把result数组作为return type，不好，消耗额外空间

public class Solution {
    /**
     * @param root: The root of the binary search tree.
     * @param k1 and k2: range k1 to k2.
     * @return: Return all keys that k1<=key<=k2 in ascending order.
     */
    public ArrayList<Integer> searchRange(TreeNode root, int k1, int k2) {
        ArrayList<Integer> res = searchRangeRecur(root,k1,k2);
        return res;
    }

    public ArrayList<Integer> searchRangeRecur(TreeNode cur, int k1, int k2){
        ArrayList<Integer> res = new ArrayList<Integer>();
        if (cur==null) return res;
        if (k1>k2) return res;

        ArrayList<Integer> left = searchRangeRecur(cur.left,k1,Math.min(cur.val-1,k2));
        ArrayList<Integer> right = searchRangeRecur(cur.right,Math.max(cur.val+1,k1),k2);

        res.addAll(left);
        if (cur.val>=k1 && cur.val<=k2) res.add(cur.val);
        res.addAll(right);

        return res;
    }
        
}

 第二遍做法：

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of the binary search tree.
     * @param k1 and k2: range k1 to k2.
     * @return: Return all keys that k1<=key<=k2 in ascending order.
     */
    public ArrayList<Integer> searchRange(TreeNode root, int k1, int k2) {
        // write your code here
        ArrayList<Integer> res = new ArrayList<Integer>();
        if (root == null || (k1 > k2)) return res;
        helper(root, k1, k2, res);
        return res;
    }
    
    public void helper(TreeNode root, int k1, int k2, ArrayList<Integer> res) {
        if (root == null) return;
        helper(root.left, k1, k2, res);
        if (k1 <= root.val && root.val <= k2) res.add(root.val);
        helper(root.right, k1, k2, res);
    }
}