Given the root node of a binary search tree, return the sum of values of all nodes with value between L and R (inclusive).
The binary search tree is guaranteed to have unique values.

Solution：分治

思路：

Time Complexity: O() Space Complexity: O()

Solution Code：

class Solution {
    public int rangeSumBST(TreeNode root, int L, int R) {
        if (root == null) {
            return 0;
        }
        
        int sum = 0;
        if (L < root.val) {
            sum += rangeSumBST(root.left, L, R);
        }
        if (root.val < R) {
            sum += rangeSumBST(root.right, L, R);
        }
        if (L <= root.val && root.val <= R) {
            sum += root.val;
        }
        
        return sum;
    }
}