[LeetCode] 307. Range Sum Query - Mutable

 Problem : https://leetcode.com/problems/range-sum-query-mutable/

Cache accumulative sum of each element and diffs introduced by each update.

class NumArray:

    def __init__(self, nums: List[int]):
        self.nums = nums
        self.sums = [0] * (len(nums) + 1)
        for i in range(len(nums)):
            self.sums[i+1] = self.sums[i] + self.nums[i]
        
        self.diff = {}
        
    def update(self, i: int, val: int) -> None:
        self.diff[i] = val - self.nums[i]
        
    def sumRange(self, i: int, j: int) -> int:
        result = self.sums[j+1] - self.sums[i]
        
        #print(self.nums, self.diff)
        
        for k in self.diff.keys():
            if i <=  k  <= j :
                result += self.diff[k]
        
        return result
        


# Your NumArray object will be instantiated and called as such:
# obj = NumArray(nums)
# obj.update(i,val)
# param_2 = obj.sumRange(i,j)