303. 区域和检索 - 数组不可变

题目：

 

思路：

【1】利用左右汇总数据集的方式（当然做完后你会发现单个数据集其实也是可以的，毕竟一开始我以为是乘积）

代码展示：

对数据集的优化：

//时间7 ms击败100%
//内存44.1 MB击败75.81%
class NumArray {
    private int[] sums;

    public NumArray(int[] nums) {
        sums = new int[nums.length + 1];
        for (int i = 1; i <= nums.length; i++) {
            sums[i] = sums[i - 1] + nums[i - 1];
        }
    }

    public int sumRange(int i, int j) {
        return sums[j + 1] - sums[i];
    }
}

 

利用左右汇总数据集的方式：

//时间7 ms击败100%
//内存44.5 MB击败30.47%
class NumArray {
    int[] left_nums;
    int[] right_nums;
    int sum;

    public NumArray(int[] nums) {
        left_nums = new int[nums.length];
        right_nums = new int[nums.length];

        for (int i = 0; i < nums.length; i++){
            sum += nums[i];
        }
        for (int i = 1; i < nums.length; i++){
            left_nums[i] = left_nums[i - 1] + nums[i-1];
        }
        for (int i = nums.length - 2; i >= 0; i--){
            right_nums[i] = right_nums[i + 1] + nums[i+1];
        }
    }
    
    public int sumRange(int left, int right) {
        return sum - left_nums[left] - right_nums[right];
    }
}

/**
 * Your NumArray object will be instantiated and called as such:
 * NumArray obj = new NumArray(nums);
 * int param_1 = obj.sumRange(left,right);
 */