/**
 * 对于不可变数组，用线段树反而更麻烦
 */
class NumArray {

    int[] sum;

    public NumArray(int[] nums) {

        /**
         * 提前保存前n个元素的和，sum[i]保存的是前i个元素，不包括自身
         * 区间和就是左右边界前缀和的差
         */
        sum = new int[nums.length + 1];
        sum[0] = 0;

        for (int i = 1; i < sum.length; i++) {
            sum[i] = sum[i - 1] + nums[i - 1];
        }
    }

    public int sumRange(int left, int right) {

        return sum[right + 1] - sum[left];
    }
}

https://leetcode-cn.com/problems/range-sum-query-immutable/