303. Range Sum Query - Immutable

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

Implement the NumArray class:

  • NumArray(int[] nums) Initializes the object with the integer array nums.
  • int sumRange(int i, int j) Return the sum of the elements of the nums array in the range [i, j] inclusive (i.e., sum(nums[i], nums[i + 1], ... , nums[j]))

 

Example 1:

Input
["NumArray", "sumRange", "sumRange", "sumRange"]
[[[-2, 0, 3, -5, 2, -1]], [0, 2], [2, 5], [0, 5]]
Output
[null, 1, -1, -3]

Explanation
NumArray numArray = new NumArray([-2, 0, 3, -5, 2, -1]);
numArray.sumRange(0, 2); // return 1 ((-2) + 0 + 3)
numArray.sumRange(2, 5); // return -1 (3 + (-5) + 2 + (-1)) 
numArray.sumRange(0, 5); // return -3 ((-2) + 0 + 3 + (-5) + 2 + (-1))

 

Constraints:

  • 0 <= nums.length <= 104
  • -105 <= nums[i] <= 105
  • 0 <= i <= j < nums.length
  • At most 104 calls will be made to sumRange.

303. Range Sum Query - Immutable
struct NumArray {
    prefix_sums: Vec<i32>,
}

impl NumArray {
    fn new(mut nums: Vec<i32>) -> Self {
        for i in 1..nums.len() {
            nums[i] += nums[i - 1];
        }
        NumArray { prefix_sums: nums }
    }

    fn sum_range(&self, i: i32, j: i32) -> i32 {
        let j: usize = j as usize;
        let i: usize = i as usize;
        if i == 0 {
            self.prefix_sums[j]
        } else {
            self.prefix_sums[j] - self.prefix_sums[i - 1]
        }
    }
}

#[test]
fn test() {
    let obj = NumArray::new(vec![-2, 0, 3, -5, 2, -1]);
    assert_eq!(obj.sum_range(0, 2), 1);
    assert_eq!(obj.sum_range(2, 5), -1);
    assert_eq!(obj.sum_range(0, 5), -3);
}