leetcode 303. 区域和检索 - 数组不可变

class NumArray {
    public int[] prefixSum;
    public NumArray(int[] nums) {
        prefixSum = new int[nums.length];
        prefixSum[0] = nums[0];
        for(int i = 1;i < nums.length;i++)
            prefixSum[i] = prefixSum[i - 1] + nums[i];
    }
    
    public int sumRange(int left, int right) {
        return left == 0 ? prefixSum[right] : prefixSum[right] - prefixSum[left - 1];
    }
}

引入前缀和的概念就很容易做了