rs: add «786. K-th Smallest Prime Fraction»

Signed-off-by: Matej Focko <me@mfocko.xyz>

rs/k-th-smallest-prime-fraction.rs

@@ -0,0 +1,94 @@
+use std::cmp;
+use std::collections::BinaryHeap;
+
+#[derive(Eq)]
+struct Fraction {
+    num: i32,
+    den: i32,
+    num_idx: usize,
+    den_idx: usize,
+}
+
+impl Fraction {
+    fn new(primes: &[i32], num_idx: usize, den_idx: usize) -> Self {
+        Self {
+            num: primes[num_idx],
+            den: primes[den_idx],
+            num_idx,
+            den_idx,
+        }
+    }
+}
+
+impl Ord for Fraction {
+    fn cmp(&self, rhs: &Fraction) -> cmp::Ordering {
+        let left = self.num * rhs.den;
+        let right = rhs.num * self.den;
+
+        left.cmp(&right)
+    }
+}
+
+impl PartialOrd for Fraction {
+    fn partial_cmp(&self, rhs: &Fraction) -> Option<cmp::Ordering> {
+        Some(self.cmp(rhs))
+    }
+}
+
+impl PartialEq for Fraction {
+    fn eq(&self, rhs: &Fraction) -> bool {
+        self.num * rhs.den == rhs.num * self.den
+    }
+}
+
+impl From<Fraction> for Vec<i32> {
+    fn from(f: Fraction) -> Self {
+        vec![f.num, f.den]
+    }
+}
+
+struct Solution {}
+impl Solution {
+    pub fn kth_smallest_prime_fraction(arr: Vec<i32>, k: i32) -> Vec<i32> {
+        let mut heap: BinaryHeap<cmp::Reverse<Fraction>> = BinaryHeap::new();
+
+        let last = arr.len() - 1;
+        for i in 0..arr.len() {
+            heap.push(cmp::Reverse(Fraction::new(&arr, i, last)));
+        }
+
+        for _ in 1..k {
+            let smallest = heap.pop().expect("There is always at least one fraction").0;
+            if smallest.den_idx - 1 > smallest.num_idx {
+                heap.push(cmp::Reverse(Fraction::new(
+                    &arr,
+                    smallest.num_idx,
+                    smallest.den_idx - 1,
+                )));
+            }
+        }
+
+        heap.pop().expect("There is always k-th smallest").0.into()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn example_1() {
+        assert_eq!(
+            Solution::kth_smallest_prime_fraction(vec![1, 2, 3, 5], 3),
+            vec![2, 5]
+        );
+    }
+
+    #[test]
+    fn example_2() {
+        assert_eq!(
+            Solution::kth_smallest_prime_fraction(vec![1, 7], 1),
+            vec![1, 7]
+        );
+    }
+}