Leetcode_1395. 统计作战单位数

求升序或降序三元组的数量

bit求出每个数两侧大于和小于的数的个数，然后枚举三元组中间数字。

code

class Solution {
public:
    int c[100050];
    int lowbit(int x){
        return x&(-x);
    }
    void add(int i,int x){
        int n=int(1e5);
        while(i<=n){
            c[i]+=x;
            i+=lowbit(i);
        }
    }
    int sum(int i){
        int ans=0;
        int n=int(1e5);
        while(i){
            ans+=c[i];
            i-=lowbit(i);
        }
        return ans;
    }
    int numTeams(vector<int>& rating) {
        int n=rating.size();
        vector<int> les(n,0),rib(n,0),leb(n,0),ris(n,0);
        int ans=0;
        for(int i=n-1;i>=0;i--){
            ris[i]=sum(rating[i]);
            rib[i]=(n-1-i)-ris[i];
            add(rating[i],1);
        }
        memset(c,0,sizeof(c));
        for(int i=0;i<n;i++){
            les[i]=sum(rating[i]);
            leb[i]=i-les[i];
            add(rating[i],1);
        }
        for(int i=0;i<n;i++){
            ans+=les[i]*rib[i];
            ans+=leb[i]*ris[i];
        }
        return ans;
    }
};