Range Sum Query - Immutable

Question

Given an integer array nums, find the sum of the elements between indices i and j (i ≤ j), inclusive.

Example:

Given nums = [-2, 0, 3, -5, 2, -1]

sumRange(0, 2) -> 1
sumRange(2, 5) -> -1
sumRange(0, 5) -> -3

Note:

1. You may assume that the array does not change.
2. There are many calls to sumRange function.

Tags

• Array
• Dynamic Programming

Thought

Use an array to store the sum of the subarray from the start to the current position in the array when initialization.

Code

class NumArray(object):

    def __init__(self, nums):
        """
        :type nums: List[int]
        """
        self.sumArray = [0]
        for num in nums:
            self.sumArray.append(self.sumArray[-1] + num)

    def sumRange(self, i, j):
        """
        :type i: int
        :type j: int
        :rtype: int
        """
        return self.sumArray[j + 1] - self.sumArray[i]


# Your NumArray object will be instantiated and called as such:
# obj = NumArray(nums)
# param_1 = obj.sumRange(i,j)