From 43d043b5e1530ce91ba7ed2314bfac3749e7ff6b Mon Sep 17 00:00:00 2001
From: Matej Focko <mfocko@redhat.com>
Date: Sun, 16 Apr 2023 00:05:53 +0200
Subject: [PATCH] =?UTF-8?q?problems(rs):=20add=20=E2=80=9C303.=20Range=20S?=
 =?UTF-8?q?um=20Query=20-=20Immutable=E2=80=9D?=
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Signed-off-by: Matej Focko <mfocko@redhat.com>
---
 problems/range-sum-query-immutable.rs | 45 +++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)
 create mode 100644 problems/range-sum-query-immutable.rs

diff --git a/problems/range-sum-query-immutable.rs b/problems/range-sum-query-immutable.rs
new file mode 100644
index 0000000..1aa4dd8
--- /dev/null
+++ b/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);
+    }
+}