思路

计算前缀和

class NumArray {
    private int[] sum;
    public NumArray(int[] nums) {
        sum = new int[nums.length+1];
        //计算前缀和
        for(int i=0;i<nums.length;i++)
            sum[i+1] = sum[i]+nums[i];
    }
    public int sumRange(int i, int j) {
        //用前缀和进行求区间和
        return sum[j+1]-sum[i];
    }
}