Create 2563. Count the Number of Fair Pairs1

2563. Count the Number of Fair Pairs1

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+class Solution {
+public:
+    long long countFairPairs(vector<int>& nums, int lower, int upper) {
+        sort(nums.begin(), nums.end());
+        return lower_bound(nums, upper + 1) - lower_bound(nums, lower);
+    }
+
+private:
+    long long lower_bound(vector<int>& nums, int value) {
+        int left = 0, right = nums.size() - 1;
+        long long result = 0;
+        while (left < right) {
+            int sum = nums[left] + nums[right];
+            if (sum < value) {
+                result += (right - left);
+                left++;
+            } else {
+                right--;
+            }
+        }
+        return result;
+    }
+};