add 0493 folder & cpp vserion

Author: zouwx2cs

solution/0493.Reverse Pairs/Solution.cpp

@@ -0,0 +1,79 @@
+class Solution {
+private:
+    inline bool cmp1(vector<int>::iterator a,
+                    vector<int>::iterator b)
+    {
+        if (*a >= 0 && *b >= 0)
+        {
+            const unsigned ua = *a, ub = *b ;
+            return ua <= (ub << 1) ;
+        }
+        
+        if (*a < 0 && *b < 0)
+        {
+            const unsigned ua = -*a, ub = -*b ;
+            return ua >= (ub << 1) ;
+        }
+        
+        if (*a < 0 && *b >= 0)
+            return true ;
+        
+        if (*a >= 0 && *b < 0)
+            return false ;
+    }
+
+    unsigned CountLRPairs(vector<int>::iterator start,
+                          vector<int>::iterator end)
+    {
+        if (start + 1 >= end)
+            return 0 ;
+        if (start + 2 == end)
+        {
+            if (*start <= *(start+1))
+                return !cmp1(start, start+1) ; // 这里也要计算，应为可能有负数
+            else // 这边也要排序，需要特别注意排序的条件是a[i] < a[j]，而逆序的条件是a[i] < 2*a[j]
+            {
+                swap(*start, *(start+1)) ;
+                return !cmp1(start+1, start) ; // 交换过位置了
+            }
+        }
+
+        vector<int>::iterator mid = start + ((end-start) >> 1) ;
+        vector<int>::iterator pF = start, pB = mid ;
+        int cnt = CountLRPairs(start, mid) + CountLRPairs(mid, end) ;
+        while (pF != mid)
+        {
+            while (pB != end && !cmp1(pF, pB))
+                ++pB ;
+            cnt += pB-mid ;
+            ++pF ;
+        }
+
+        vector<int> tmp(start, mid) ;
+        pF = tmp.begin() ;
+        pB = mid ;
+        vector<int>::iterator p = start ;
+
+        auto size = end - start ;
+        if (size == 10)
+            ++size ;
+
+        while (pF != tmp.end() && pB != end)
+        {
+            if (*pF <= *pB)
+                *p++ = *pF++ ;
+            else
+                *p++ = *pB++ ;
+        }
+        while (pF != tmp.end())
+            *p++ = *pF++ ;
+        while (pB != end)
+            *p++ = *pB++ ;
+
+        return cnt ;
+    }
+public:
+    int reversePairs(vector<int>& nums) {
+        return CountLRPairs(nums.begin(), nums.end()) ;
+    }
+};