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);
}
``````