reverse pairs

Author: sachuverma

Striver Sheet/Day-3/Reverse Pairs.cpp

@@ -0,0 +1,80 @@
+/*
+Reverse Pairs
+=============
+
+Given an integer array nums, return the number of reverse pairs in the array.
+
+A reverse pair is a pair (i, j) where 0 <= i < j < nums.length and nums[i] > 2 * nums[j].
+
+Example 1:
+Input: nums = [1,3,2,3,1]
+Output: 2
+
+Example 2:
+Input: nums = [2,4,3,5,1]
+Output: 3
+
+Constraints:
+1 <= nums.length <= 5 * 104
+231 <= nums[i] <= 231 - 1
+*/
+
+class Solution
+{
+public:
+  int merge(vector<int> &nums, int low, int mid, int high)
+  {
+    int count = 0;
+    int i = low, j = mid + 1;
+
+    // count pairs
+    while (i <= mid)
+    {
+      while (j <= high && nums[i] > 2LL * nums[j])
+        j++;
+      count += (j - (mid + 1));
+      i++;
+    }
+
+    // merge the sorted arrays
+    vector<int> temp;
+    i = low;
+    j = mid + 1;
+
+    while (i <= mid && j <= high)
+    {
+      if (nums[i] < nums[j])
+        temp.push_back(nums[i++]);
+      else
+        temp.push_back(nums[j++]);
+    }
+
+    while (i <= mid)
+      temp.push_back(nums[i++]);
+    while (j <= high)
+      temp.push_back(nums[j++]);
+
+    for (int k = low; k <= high; ++k)
+      nums[k] = temp[k - low];
+
+    return count;
+  }
+
+  int mergeSort(vector<int> &nums, int low, int high)
+  {
+    int count = 0;
+    if (low < high)
+    {
+      int mid = (low + high) / 2;
+      count += mergeSort(nums, low, mid);
+      count += mergeSort(nums, mid + 1, high);
+      count += merge(nums, low, mid, high);
+    }
+    return count;
+  }
+
+  int reversePairs(vector<int> &nums)
+  {
+    return mergeSort(nums, 0, nums.size() - 1);
+  }
+};

Striver Sheet/README.md

@@ -27,7 +27,7 @@
 - [Majority Element](https://leetcode.com/problems/majority-element/) - [Cpp Soultion](./Day-3/Majority%20Element.cpp)
 - [Majority Element II](https://leetcode.com/problems/majority-element-ii/) - [Cpp Soultion](./Day-3/Majority%20Element%20II.cpp)
 - [Unique Paths](https://leetcode.com/problems/unique-paths/) - [Cpp Soultion](./Day-3/Unique%20Paths.cpp)
-- []() - [Cpp Soultion](./Day-3/.cpp)
+- [Reverse Pairs](https://leetcode.com/problems/reverse-pairs/) - [Cpp Soultion](./Day-3/Reverse%20Pairs.cpp)
 
 ###