二叉查找树中搜索区间

链接：http://www.lintcode.com/zh-cn/problem/search-range-in-binary-search-tree/

 * 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: param root: The root of the binary search tree
     * @param k1: An integer
     * @param k2: An integer
     * @return: return: Return all keys that k1<=key<=k2 in ascending order
     */
    public List<Integer> searchRange(TreeNode root, int k1, int k2) {
        // write your code here
        
        List<Integer> result = new ArrayList<>();
        if (root == null) {
            return null;
        }
        //递归定义
        //递归拆解
        if (root.left != null) {
            
                result.addAll(searchRange(root.left, k1, k2));
            
        }
       
        if (root.val >= k1 && root.val <= k2) {
            result.add(root.val);
        }
        if(root.right != null) {
            
                result.addAll(searchRange(root.right,k1, k2));
        }
        
        return result;
        //递归出口
        
    }
}

本题使用递归方式解决，主要找到递归的拆解以及出口
需要注意的是对null值得判断