思路：

bit + 离散化。

实现：

1 #include 
     
       2 using namespace std; 3 class Solution  4 { 5 public: 6     int sum(vector
      
        & bit, int i) 7     { 8         int ans = 0; 9         while (i) { ans += bit[i]; i -= i & -i; }10         return ans;11     }12     void add(vector
       
         & bit, int i, int x)13     {14         int n = bit.size() - 1;15         while (i <= n) { bit[i] += x; i += i & -i; }16     }17     vector
        
          countSmaller(vector
         
          & nums) 18     {19         int n = nums.size();20         vector
          
            a(nums.begin(), nums.end()), b(nums.begin(), nums.end());21 sort(a.begin(), a.end());22 a.erase(unique(a.begin(), a.end()), a.end());23 for (int i = 0; i < n; i++)24 b[i] = lower_bound(a.begin(), a.end(), b[i]) - a.begin() + 1;25 vector
           
             bit(n + 1, 0), ans(n, 0);26 for (int i = n - 1; i >= 0; i--)27 {28 add(bit, b[i], 1);29 ans[i] = b[i] == 1 ? 0 : sum(bit, b[i] - 1);30 }31 return ans;32 }33 };34 int main()35 {36 vector
            
              v;37 v.push_back(5);38 v.push_back(-1);39 v.push_back(6);40 v.push_back(1);41 vector
             
               ans = Solution().countSmaller(v);42 for (auto it: ans) cout << it << " ";43 cout << endl;44 return 0;45 }