Leetcode 303. Range Sum Query - Immutable

Problem

Given an integer array nums, handle multiple queries of the following type:

Calculate the sum of the elements of nums between indices left and right inclusive where left <= right.
Implement the NumArray class:

• NumArray(int[] nums) Initializes the object with the integer array nums.
• int sumRange(int left, int right) Returns the sum of the elements of nums between indices left and right inclusive (i.e. nums[left] + nums[left + 1] + … + nums[right]).

Algorithm

Dynamic Programming (DP). Use a list sums to save the sum from 0 to i, and sumRange(left, right) = sums[right] - sums[left-1]

Code

class NumArray:

    def __init__(self, nums: List[int]):
        nlen = len(nums)
        self.sums = [nums[0]] * nlen
        for i in range(1, nlen):
            self.sums[i] = self.sums[i-1] + nums[i]

    def sumRange(self, left: int, right: int) -> int:
        if left <= 0:
            return self.sums[right]
        else:
            return self.sums[right] - self.sums[left-1]