题目：
题解：
func countRangeSum(nums []int, lower, upper int) int {var mergeCount func([]int) intmergeCount = func(arr []int) int {n := len(arr)if n <= 1 {return 0}n1 := append([]int(nil), arr[:n/2]...)n2 := append([]int(nil), arr[n/2:]...)cnt := mergeCount(n1) + mergeCount(n2) // 递归完毕后，n1 和 n2 均为有序// 统计下标对的数量l, r := 0, 0for _, v := range n1 {for l < len(n2) && n2[l]-v < lower {l++}for r < len(n2) && n2[r]-v <= upper {r++}cnt += r - l}// n1 和 n2 归并填入 arrp1, p2 := 0, 0for i := range arr {if p1 < len(n1) && (p2 == len(n2) || n1[p1] <= n2[p2]) {arr[i] = n1[p1]p1++} else {arr[i] = n2[p2]p2++}}return cnt}prefixSum := make([]int, len(nums)+1)for i, v := range nums {prefixSum[i+1] = prefixSum[i] + v}return mergeCount(prefixSum)
}