303. Range Sum Query - Immutable

dynamic programming 
status array d[i] represent the sum of all the numbers before the i th (not including i) 
class NumArray(object):
    def __init__(self, nums):
        """
        initialize your data structure here.
        :type nums: List[int]
        """
        self.accu=[0]
        for num in nums:
            self.accu.append(self.accu[-1]+num)

    def sumRange(self, i, j):
        """
        sum of elements nums[i..j], inclusive.
        :type i: int
        :type j: int
        :rtype: int
        """
        return self.accu[j+1]-self.accu[i]
        


# Your NumArray object will be instantiated and called as such:
# numArray = NumArray(nums)
# numArray.sumRange(0, 1)
# numArray.sumRange(1, 2)