add reverse pairs

Author: boudhayan

Algorithm Solutions In Swift.xcodeproj/project.pbxproj

@@ -8,6 +8,7 @@
 
 /* Begin PBXBuildFile section */
 		1F21EEE72F54613100E0274E /* MakeArithmeticProgression.swift in Sources */ = {isa = PBXBuildFile; fileRef = 1F21EEE62F54612300E0274E /* MakeArithmeticProgression.swift */; };
+		1F21EEEB2F56CA9700E0274E /* ReversePairs.swift in Sources */ = {isa = PBXBuildFile; fileRef = 1F21EEEA2F56CA9300E0274E /* ReversePairs.swift */; };
 		1F7037522F3E5618002CFFE1 /* Algorithms in Frameworks */ = {isa = PBXBuildFile; productRef = 1F7037512F3E5618002CFFE1 /* Algorithms */; };
 		1F7037552F3E5642002CFFE1 /* BasicContainers in Frameworks */ = {isa = PBXBuildFile; productRef = 1F7037542F3E5642002CFFE1 /* BasicContainers */; };
 		1F7037572F3E5642002CFFE1 /* BitCollections in Frameworks */ = {isa = PBXBuildFile; productRef = 1F7037562F3E5642002CFFE1 /* BitCollections */; };
@@ -334,6 +335,7 @@
 
 /* Begin PBXFileReference section */
 		1F21EEE62F54612300E0274E /* MakeArithmeticProgression.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = MakeArithmeticProgression.swift; sourceTree = "<group>"; };
+		1F21EEEA2F56CA9300E0274E /* ReversePairs.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ReversePairs.swift; sourceTree = "<group>"; };
 		1F85E42D2F500D060097933E /* RecursiveBubbleSort.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = RecursiveBubbleSort.swift; sourceTree = "<group>"; };
 		1F85E4302F500D210097933E /* RecusriveInsertionSort.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = RecusriveInsertionSort.swift; sourceTree = "<group>"; };
 		1F85E4352F5062420097933E /* SortedRotated.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = SortedRotated.swift; sourceTree = "<group>"; };
@@ -608,6 +610,21 @@
 			path = "Can Make Arithmetic Progression";
 			sourceTree = "<group>";
 		};
+		1F21EEE82F56CA7100E0274E /* Reverse Pairs */ = {
+			isa = PBXGroup;
+			children = (
+				1F21EEEA2F56CA9300E0274E /* ReversePairs.swift */,
+			);
+			path = "Reverse Pairs";
+			sourceTree = "<group>";
+		};
+		1F21EEE92F56CA8D00E0274E /* Next Permutation */ = {
+			isa = PBXGroup;
+			children = (
+			);
+			path = "Next Permutation";
+			sourceTree = "<group>";
+		};
 		1F85E42C2F500D010097933E /* Recursive Bubble Sort */ = {
 			isa = PBXGroup;
 			children = (
@@ -1361,6 +1378,8 @@
 				6A88CEE626C056BD00A3746B /* 3 Sum Closest */,
 				1F85E42F2F500D1D0097933E /* Recursive Insertion Sort */,
 				1F21EEE52F54611E00E0274E /* Can Make Arithmetic Progression */,
+				1F21EEE82F56CA7100E0274E /* Reverse Pairs */,
+				1F21EEE92F56CA8D00E0274E /* Next Permutation */,
 			);
 			path = LeetCode;
 			sourceTree = "<group>";
@@ -2936,6 +2955,7 @@
 				6A91246127064A8900D50C17 /* MaxAreaOfIsland.swift in Sources */,
 				6A319EC02908764100801A8A /* SymmetricTree_recursive.swift in Sources */,
 				6A651A25290705FB000731BB /* ValidParantheses.swift in Sources */,
+				1F21EEEB2F56CA9700E0274E /* ReversePairs.swift in Sources */,
 				6AF5789226A2E6F6007B37CA /* NonConstructibleChange.swift in Sources */,
 				6A651A46290785A2000731BB /* ValidSuduku.swift in Sources */,
 				6A88CEE826C056C900A3746B /* ThreeSumClosest.swift in Sources */,

Algorithm Solutions In Swift/main.swift

@@ -2,5 +2,3 @@ import Foundation
 
 
 
-var array = [3, 5, 7]
-print(canMakeArithmeticProgression(array))

LeetCode/Reverse Pairs/ReversePairs.swift

@@ -0,0 +1,65 @@
+//
+//  ReversePairs.swift
+//  Algorithm Solutions In Swift
+//
+//  Created by littlebanana on 03/03/26.
+//
+
+func reversePairs(_ nums: [Int]) -> Int {
+    var arr = nums
+    return mergeSort(&arr, low: 0, high: nums.count - 1)
+}
+
+func mergeSort(_ nums: inout [Int], low: Int, high: Int) -> Int {
+    var count = 0
+    if low >= high { return count }
+    let mid = (low + high) / 2
+    count += mergeSort(&nums, low: low, high: mid)
+    count += mergeSort(&nums, low: mid + 1, high: high)
+    count += countPairs(nums: nums, low: low, mid: mid, high: high)
+    merge(nums: &nums, low: low, mid: mid, high: high)
+    return count
+}
+
+func countPairs(nums: [Int], low: Int, mid: Int, high: Int) -> Int {
+    var right = mid + 1
+    var count = 0
+    for i in low...mid {
+        while right <= high, nums[i] > 2 * nums[right] {
+            right += 1
+        }
+        count += right - (mid + 1)
+    }
+    return count
+}
+
+func merge(nums: inout [Int], low: Int, mid: Int, high: Int) {
+    var i = low, j = mid + 1
+    var temp = [Int]()
+    while i <= mid, j <= high {
+        if nums[i] < nums[j] {
+            temp.append(nums[i])
+            i += 1
+        } else {
+            temp.append(nums[j])
+            j += 1
+        }
+    }
+    while i <= mid {
+        temp.append(nums[i])
+        i += 1
+    }
+    while j <= high {
+        temp.append(nums[j])
+        j += 1
+    }
+    var k = low
+    for i in 0..<temp.count {
+        nums[k] = temp[i]
+        k += 1
+    }
+}
+
+
+//var arr = [2,4,3,5,1]
+//print(reversePairs(arr))