subarray sums 2 solution

Signed-off-by: Lance-Drane <ldraneutk@gmail.com>

src/bin/search_subarray_sums_1.rs

@@ -53,7 +53,7 @@ impl UnsafeScanner<'_> {
 /// <b>Constraints</b>
 ///
 /// <ul>
-/// <li>1 ≤ n, q ≤ 2 * 10<sup>5</sup></li>
+/// <li>1 ≤ n, x ≤ 2 * 10<sup>5</sup></li>
 /// <li>1 ≤ x,a<sub>i</sub> ≤ 10<sup>9</sup></li>
 /// </ul>
 fn solve<W: std::io::Write>(mut scan: UnsafeScanner, out: &mut W) {

src/bin/search_subarray_sums_2.rs

@@ -0,0 +1,125 @@
+// I/O boilerplate //
+
+pub struct UnsafeScanner<'a> {
+    // not actually dead code, needed for buf_iter to work
+    #[allow(dead_code)]
+    buf_str: Vec<u8>,
+    buf_iter: std::str::SplitAsciiWhitespace<'a>,
+}
+
+impl UnsafeScanner<'_> {
+    pub fn new<R: std::io::Read>(mut reader: R) -> Self {
+        let mut buf_str = vec![];
+        unsafe {
+            reader.read_to_end(&mut buf_str).unwrap_unchecked();
+        }
+        let buf_iter = unsafe {
+            let slice = std::str::from_utf8_unchecked(&buf_str);
+            std::mem::transmute(slice.split_ascii_whitespace())
+        };
+
+        Self { buf_str, buf_iter }
+    }
+
+    /// Use "turbofish" syntax `token::<T>()` to select data type of next token.
+    ///
+    /// # Panics
+    /// Panics if there's no more tokens or if the token cannot be parsed as T.
+    pub fn token<T: std::str::FromStr>(&mut self) -> T {
+        unsafe {
+            self.buf_iter
+                .next()
+                .unwrap_unchecked()
+                .parse()
+                .unwrap_unchecked()
+        }
+    }
+}
+
+// problem //
+
+use std::collections::HashMap;
+
+/// Given an array of n integers, your task is to count the number of subarrays having sum x.
+///
+/// <b>Input</b>
+///
+/// The first input line has two integers n and x: the size of the array and the target sum x.
+///
+/// The next line has n integers a<sub>1</sub>,a<sub>2</sub>,...,a<sub>n</sub>: the contents of the array.
+///
+/// <b>Output</b>
+///
+/// Print one integer: the required number of subarrays.
+///
+/// <b>Constraints</b>
+///
+/// <ul>
+/// <li>1 ≤ n, x ≤ 2 * 10<sup>5</sup></li>
+/// <li>-10<sup>9</sup> ≤ x,a<sub>i</sub> ≤ 10<sup>9</sup></li>
+/// </ul>
+fn solve<W: std::io::Write>(mut scan: UnsafeScanner, out: &mut W) {
+    let n: usize = scan.token();
+    let target: i64 = scan.token();
+    let mut counter = 0_u64;
+    let mut sum = 0_i64;
+    let mut map = HashMap::with_capacity(n);
+    map.insert(0, 1);
+
+    for num in (0..n).map(|_| scan.token::<i64>()) {
+        sum += num;
+        if let Some(prev) = map.get(&(sum - target)) {
+            counter += prev;
+        }
+        map.entry(sum).and_modify(|e| *e += 1).or_insert(1);
+    }
+
+    writeln!(out, "{counter}").unwrap();
+}
+
+// entrypoints //
+
+fn main() {
+    let scan = UnsafeScanner::new(std::io::stdin());
+    let mut out = std::io::BufWriter::new(std::io::stdout().lock());
+    solve(scan, &mut out);
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    fn test(input: &[u8], target: &[u8]) {
+        let scan = UnsafeScanner::new(input);
+        let mut out = Vec::with_capacity(target.len());
+        solve(scan, &mut out);
+
+        assert_eq!(out, target);
+    }
+
+    #[test]
+    fn test_example() {
+        let input = b"\
+5 7
+2 -1 3 5 -2
+";
+        let target = b"\
+2
+";
+
+        test(input, target);
+    }
+
+    #[test]
+    fn test_example_2() {
+        let input = b"\
+5 3
+1 1 1 -1 1
+";
+        let target = b"\
+2
+";
+
+        test(input, target);
+    }
+}