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/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func rangeSumBST(root *TreeNode, L int, R int) int {
    sum := 0
    if root != nil {
        if root.Val > L {
            sum += rangeSumBST(root.Left, L, R)   
        }
        if root.Val >= L && root.Val <= R {
            sum += root.Val
        }
        if root.Val < R {
            sum += rangeSumBST(root.Right, L, R)
        }
    }
    return sum
}