面试题51. 数组中的逆序对

class Solution {
public:
    int lowbit(int x) {return x&(-x);}
    void update(int i,vector& c)
    {//单点更新，更新这个点还要更新他的父亲所以是i+=lowbit(i)
        while(i <= c.size())
        {
            c[i]+=1;
            i+=lowbit(i);
        }
    }
    int getsum(int i,vector& c)
    {//获取前缀和，前缀和肯定是比i小的索引，所以i-=lowbit(i)；
        int res=0;
        while(i)
        {
            res+=c[i];
            i-=lowbit(i);
        }
        return res;
    }
    int reversePairs(vector& nums) {
        int n=nums.size();
        vector vt;
        vector c(n+20);
        for(auto i:nums) vt.push_back(i);
        sort(vt.begin(),vt.end());
        vt.erase(unique(vt.begin(),vt.end()),vt.end());
        unordered_map mp;
        int cont=1;
        for(auto i:vt) mp[i]=cont++;
        int ans=0;
        for(int i=0;i