LeetCode 315. Count of Smaller Numbers After Self(线段树，树状数组)

Posted 2021-03-05 Shendu.cc

题目

题意：找到数组里每个元素的右边有多少个元素小于当前元素

题解：单点更新，区间查询。线段树或者树状数组都可以。注意要离散化

class Solution {
public:
    int c[100005];
    int n;
    map<int,int> m;
    vector<int> countSmaller(vector<int>& nums) {
        
        vector<int> a = nums;
        sort(a.begin(),a.end());
        for(int i=0;i<a.size();i++)
        {
            m[a[i]]=i+1;
        }
        n=nums.size();
        vector<int> ans;
        
        for(int i=n-1;i>=0;i--)
        {
            ans.push_back(sum(m[nums[i]]-1));
            update(m[nums[i]]);
        }
        
        reverse(ans.begin(),ans.end());
        return ans;
            
    }
    
    int lowbit(int x)
    {
        return x&(-x);
    }
    
    void update(int pos)
    {
        while(pos<=n)
        {
            c[pos]++;
            pos+=lowbit(pos);
        }
    }
    
    int sum(int pos)
    {
        int ans=0;
        while(pos>0)
        {
            ans+=c[pos];
            pos-=lowbit(pos);
        }
        return ans;
    }
};