problems(rs): add „188. Best Time to Buy and Sell Stock IV“

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

problems/best-time-to-buy-and-sell-stock-iv.rs

@@ -0,0 +1,102 @@
+#[derive(Debug)]
+struct State {
+    price: i32,
+    profit: i32,
+}
+
+impl State {
+    fn new() -> State {
+        State {
+            price: i32::MAX,
+            profit: 0,
+        }
+    }
+
+    fn update(&self, previous: &State, price: i32) -> State {
+        let new_price = std::cmp::min(self.price, price - previous.profit);
+
+        State {
+            price: new_price,
+            profit: std::cmp::max(self.profit, price - new_price),
+        }
+    }
+}
+
+impl Solution {
+    pub fn max_profit(k: i32, prices: Vec<i32>) -> i32 {
+        let mut states: Vec<State> = (0..=k).map(|_| State::new()).collect();
+
+        prices.iter().for_each(|p| {
+            let mut new_states = Vec::from([State::new()]);
+
+            new_states.extend(
+                states
+                    .iter()
+                    .zip(states.iter().skip(1))
+                    .map(|(prev_state, state)| state.update(prev_state, *p)),
+            );
+
+            states = new_states;
+        });
+
+        states.last().expect("No state is present").profit
+    }
+}
+struct Solution {}
+
+fn main() {
+    println!("Hello World!");
+}
+
+#[cfg(test)]
+mod tests_max_profit {
+    use super::*;
+
+    #[test]
+    fn test_small_example() {
+        assert_eq!(Solution::max_profit(2, vec![2, 4, 1]), 2);
+    }
+
+    #[test]
+    fn test_bigger_example() {
+        assert_eq!(Solution::max_profit(2, vec![3, 2, 6, 5, 0, 3]), 7);
+    }
+
+    #[test]
+    fn test_small_no_transaction() {
+        assert_eq!(Solution::max_profit(0, vec![2, 4, 1]), 0);
+    }
+
+    #[test]
+    fn test_small_one_transaction() {
+        assert_eq!(Solution::max_profit(1, vec![2, 4, 1]), 2);
+    }
+
+    #[test]
+    fn test_bigger_one_transaction() {
+        assert_eq!(Solution::max_profit(1, vec![3, 2, 6, 5, 0, 3]), 4);
+    }
+
+    #[test]
+    fn test_big_all_transactions() {
+        assert_eq!(
+            Solution::max_profit(10, vec![3, 2, 6, 5, 0, 3, 1, 13, 5, 2, 1, 9, 4, 5, 69, 42]),
+            92
+        );
+    }
+
+    #[test]
+    fn test_buy_expensive() {
+        assert_eq!(Solution::max_profit(2, vec![2, 3, 14, 15]), 13);
+    }
+
+    #[test]
+    fn test_accumulates_profit() {
+        assert_eq!(Solution::max_profit(5, vec![1, 2, 1, 2, 1, 2, 1, 2, 5]), 7);
+    }
+
+    #[test]
+    fn test_no_profit_made() {
+        assert_eq!(Solution::max_profit(2, vec![5, 4]), 0);
+    }
+}