20250419 finished.

src/problem/mod.rs

@@ -607,3 +607,5 @@ mod p2537_count_the_number_of_good_subarrays;
 mod p2176_count_equal_and_divisible_pairs_in_an_array;
 
 mod p2364_count_number_of_bad_pairs;
+
+mod p2563_count_the_number_of_fair_pairs;

src/problem/p2563_count_the_number_of_fair_pairs.rs

@@ -0,0 +1,35 @@
+/**
+ * [2563] Count the Number of Fair Pairs
+ */
+pub struct Solution {}
+
+// submission codes start here
+
+impl Solution {
+    pub fn count_fair_pairs(mut nums: Vec<i32>, lower: i32, upper: i32) -> i64 {
+        nums.sort_unstable();
+        let mut result = 0;
+
+        for i in 0..nums.len() {
+            let l = nums[0..i].partition_point(|&x| x + nums[i] < lower);
+            let r = nums[0..i].partition_point(|&x| x + nums[i] <= upper);
+
+            result += (r - l);
+        }
+
+        result as i64
+    }
+}
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_2563() {
+        assert_eq!(6, Solution::count_fair_pairs(vec![0, 1, 7, 4, 4, 5], 3, 6));
+        assert_eq!(1, Solution::count_fair_pairs(vec![1, 7, 9, 2, 5], 11, 11));
+    }
+}