Create 2563. Count the Number of Fair Pairs

2563. Count the Number of Fair Pairs

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+class Solution {
+public:
+    // Prateek
+    long long countFairPairs(vector<int>& arr, int lower, int upper) {
+        // Declare the answer variable  
+        long long ans=0;
+        int n=arr.size();
+        sort(arr.begin(),arr.end());
+        //sort the array so that we can apply binary search        
+        // for every i we have to findits lower and upper
+        // since (a,b),(b,a) will be counted as 1 only 
+        // we can apply binary seach from i+1 
+        for(int i=0;i<n;i++){
+            // left and right are the left and right indexes for every i 
+            int l=i+1,r=n-1,left=-1,right=-1;
+            while(l<=r){
+                int mid=(l+r)/2;
+                if(arr[i]+arr[mid]>=lower){
+                    left=mid;
+                    r=mid-1;
+                }
+                else
+                l=mid+1;
+            }
+            l=i+1,r=n-1;
+            while(l<=r){
+                 int mid=(l+r)/2;
+                if(arr[i]+arr[mid]<=upper){
+                    right=mid;
+                    l=mid+1;
+                }
+                else
+                r=mid-1;
+            }
+            if(left!=-1&&right!=-1){
+            ans+=(right-left+1);}
+
+        }
+        return ans;
+    }
+};