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

 1 public class Solution {
 2     /**
 3      * @param root: The root of the binary search tree.
 4      * @param k1 and k2: range k1 to k2.
 5      * @return: Return all keys that k1<=key<=k2 in ascending order.
 6      */
 7     public ArrayList<Integer> searchRange(TreeNode root, int k1, int k2) {
 8         ArrayList<Integer> res = searchRangeRecur(root,k1,k2);
 9         return res;
10     }
11 
12     public ArrayList<Integer> searchRangeRecur(TreeNode cur, int k1, int k2){
13         ArrayList<Integer> res = new ArrayList<Integer>();
14         if (cur==null) return res;
15         if (k1>k2) return res;
16 
17         ArrayList<Integer> left = searchRangeRecur(cur.left,k1,Math.min(cur.val-1,k2));
18         ArrayList<Integer> right = searchRangeRecur(cur.right,Math.max(cur.val+1,k1),k2);
19 
20         res.addAll(left);
21         if (cur.val>=k1 && cur.val<=k2) res.add(cur.val);
22         res.addAll(right);
23 
24         return res;
25     }
26         
27 }

第二遍做法：

 1 /**
 2  * Definition of TreeNode:
 3  * public class TreeNode {
 4  *     public int val;
 5  *     public TreeNode left, right;
 6  *     public TreeNode(int val) {
 7  *         this.val = val;
 8  *         this.left = this.right = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     /**
14      * @param root: The root of the binary search tree.
15      * @param k1 and k2: range k1 to k2.
16      * @return: Return all keys that k1<=key<=k2 in ascending order.
17      */
18     public ArrayList<Integer> searchRange(TreeNode root, int k1, int k2) {
19         // write your code here
20         ArrayList<Integer> res = new ArrayList<Integer>();
21         if (root == null || (k1 > k2)) return res;
22         helper(root, k1, k2, res);
23         return res;
24     }
25     
26     public void helper(TreeNode root, int k1, int k2, ArrayList<Integer> res) {
27         if (root == null) return;
28         helper(root.left, k1, k2, res);
29         if (k1 <= root.val && root.val <= k2) res.add(root.val);
30         helper(root.right, k1, k2, res);
31     }
32 }