解决思路

class NumArray {
private int[] sum;
public NumArray(int[] nums) {
    sum = new int[nums.length + 1];
    for (int i = 0; i < nums.length; i++) {
        sum[i + 1] = sum[i] + nums[i];
    }
}
public int sumRange(int i, int j) {
    return sum[j + 1] - sum[i];
}
}