Решение

class NumArray(nums: IntArray) {
    private val pref: IntArray

    init {
        val n = nums.size
        pref = IntArray(n + 1)
        var i = 0
        var sum = 0
        while (i < n) {
            sum += nums[i]
            pref[i + 1] = sum
            i++
        }
    }

    fun sumRange(left: Int, right: Int): Int {
        return pref[right + 1] - pref[left]
    }
}