problems(rs): add “303. Range Sum Query - Immutable”

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/range-sum-query-immutable.rs

@@ -0,0 +1,45 @@
+#[derive(Debug)]
+struct NumArray {
+    prefix_sums: Vec<i32>,
+}
+
+impl NumArray {
+    fn new(nums: Vec<i32>) -> Self {
+        let mut total = nums.iter().sum::<i32>();
+
+        let mut prefix_sums: Vec<i32> = nums
+            .iter()
+            .scan(total, |total, &x| {
+                let prev = *total;
+                *total -= x;
+
+                Some(prev)
+            })
+            .collect();
+        prefix_sums.push(0);
+
+        Self { prefix_sums }
+    }
+
+    fn sum_range(&self, left: i32, right: i32) -> i32 {
+        self.prefix_sums[left as usize] - self.prefix_sums[right as usize + 1]
+    }
+}
+
+fn main() {}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn example_1() {
+        let num_arr = NumArray::new(vec![-2, 0, 3, -5, 2, -1]);
+
+        dbg!("{:?}", &num_arr);
+
+        assert_eq!(num_arr.sum_range(0, 2), 1);
+        assert_eq!(num_arr.sum_range(2, 5), -1);
+        assert_eq!(num_arr.sum_range(0, 5), -3);
+    }
+}