LC 面试题51. 数组中的逆序对

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Solution1 归并排序：

class Solution {
public:
    int reversePairs(vector<int>& nums) {
        mergesort(nums,0,nums.size()-1);
        return res;
    }
    int res=0;

    void mergesort(vector<int>& nums, int left, int right){
        if(left>=right) return;

        int mid=left+(right-left)/2;
        mergesort(nums,left,mid);
        mergesort(nums,mid+1,right);
        int j=mid+1;
        int i=left;
        
        vector<int> tmp;
        while(i<=mid && j<=right){
            if(nums[i]<=nums[j]){
                res+=j-1-(mid+1)+1;
                tmp.push_back(nums[i++]);
            }else{
                tmp.push_back(nums[j++]);
            }
        }
        while(i<=mid){
            res+=right-(mid+1)+1;
            tmp.push_back(nums[i++]);
        }
        while(j<=right){
            tmp.push_back(nums[j++]);
        }
        for(int i=left;i<=right;i++){
            nums[i]=tmp[i-left];
        }
        
    }

    
};

Solution2 FenwickTree:

class Solution {
public:
    struct Number{
        int index;
        int num;
    };
    int reversePairs(vector<int>& nums) {
        int n=nums.size();
        vector<Number> vs;
        for(int i=0;i<n;i++){
            vs.push_back({i,nums[i]});
        }
        sort(vs.begin(),vs.end(),[](Number n1, Number n2){
            return n1.num<n2.num;
        });
        vector<int> rank(n);
        int rk=0;
        int pre=INT_MIN;
        for(int i=0;i<n;i++){
            if(vs[i].num==pre) rank[vs[i].index]=rk;
            else {
                rank[vs[i].index]=i+1;
                rk=i+1;
            }
            pre=vs[i].num;
        }
        vector<int> fenwick(n+1);
        int res=0;
        for(int i=n-1;i>=0;i--){
            if(rank[i]!=1){
                res+=find(fenwick,rank[i]-1);
            }
            update(fenwick,rank[i]);
        }
        return res;
    }

    void update(vector<int>& fenwick, int idx){
        while(idx<=fenwick.size()-1){
            fenwick[idx]+=1;
            idx+=(idx&-idx);
        }
    }
    int find(vector<int>& fenwick, int idx){
        int res=0;
        while(idx>=1){
            res+=fenwick[idx];
            idx-=(idx&-idx);
        }
        return res;
    }

};