Add problem 3251: Find the Count of Monotonic Pairs II

Author: EFanZh

Notes.md

@@ -373,3 +373,7 @@ Why is this solution correct?
 ## [3250. Find the Count of Monotonic Pairs I](https://leetcode.com/problems/find-the-count-of-monotonic-pairs-i/)
 
 Implement the [combinatorics solution](https://leetcode.com/problems/find-the-count-of-monotonic-pairs-i/solutions/5633883/python3-combinatorics-explanation-on-by-ye3ow/).
+
+## [3251. Find the Count of Monotonic Pairs II](https://leetcode.com/problems/find-the-count-of-monotonic-pairs-ii/)
+
+Implement the [combinatorics solution](https://leetcode.com/problems/find-the-count-of-monotonic-pairs-i/solutions/5633883/python3-combinatorics-explanation-on-by-ye3ow/).

src/lib.rs

@@ -2156,6 +2156,7 @@ pub mod problem_3242_design_neighbor_sum_service;
 pub mod problem_3248_snake_in_matrix;
 pub mod problem_3249_count_the_number_of_good_nodes;
 pub mod problem_3250_find_the_count_of_monotonic_pairs_i;
+pub mod problem_3251_find_the_count_of_monotonic_pairs_ii;
 pub mod problem_3350_adjacent_increasing_subarrays_detection_ii;
 
 #[cfg(test)]

src/problem_3251_find_the_count_of_monotonic_pairs_ii/dynamic_programming.rs

@@ -0,0 +1,55 @@
+pub struct Solution;
+
+// ------------------------------------------------------ snip ------------------------------------------------------ //
+
+use std::mem;
+
+impl Solution {
+    pub fn count_of_pairs(nums: Vec<i32>) -> i32 {
+        const MODULUS: u32 = 1_000_000_007;
+
+        let nums = nums.into_iter().map(i32::cast_unsigned).collect::<Vec<_>>();
+        let mut iter = nums.iter();
+        let mut prev = iter.next().copied().unwrap();
+        let mut cache = (1..=prev + 1).collect::<Vec<_>>();
+        let mut temp = Vec::new();
+
+        for &num in iter {
+            let mut sum = 0;
+
+            temp.extend((0..=num).map(|split| {
+                sum += prev
+                    .checked_sub(num - split)
+                    .map_or(0, |max_prev| cache[(split.min(max_prev) as usize).min(cache.len() - 1)]);
+
+                if sum >= MODULUS {
+                    sum -= MODULUS;
+                }
+
+                sum
+            }));
+
+            cache.clear();
+            prev = num;
+            mem::swap(&mut cache, &mut temp);
+        }
+
+        cache.last().unwrap().cast_signed()
+    }
+}
+
+// ------------------------------------------------------ snip ------------------------------------------------------ //
+
+impl super::Solution for Solution {
+    fn count_of_pairs(nums: Vec<i32>) -> i32 {
+        Self::count_of_pairs(nums)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_solution() {
+        super::super::tests::run::<super::Solution>();
+    }
+}

src/problem_3251_find_the_count_of_monotonic_pairs_ii/mod.rs

@@ -0,0 +1,18 @@
+pub mod dynamic_programming;
+
+pub trait Solution {
+    fn count_of_pairs(nums: Vec<i32>) -> i32;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Solution;
+
+    pub fn run<S: Solution>() {
+        let test_cases = [(&[2, 3, 2] as &[_], 4), (&[5, 5, 5, 5], 126)];
+
+        for (nums, expected) in test_cases {
+            assert_eq!(S::count_of_pairs(nums.to_vec()), expected);
+        }
+    }
+}