LeetCode 493. 翻轉對(歸併排序 || 離散化+樹狀陣列)

493. 翻轉對

• 方法一

class Solution {
public:
    int reversePairs(vector<int>& nums) {
        return div(nums,0,nums.size()-1);
    }
    int div(vector<int> &nums,int l,int r){
        if(l>=r) return 0;
        int mid = l+(r-l)/2;
        int res = div(nums,l,mid)+div(nums,mid+1,r);
        // 合併
        int j = 0;
        for(int i=l;i<=mid;i++){
            while(j+mid+1<=r && (long)nums[i]>(long)2*nums[j+mid+1]){
                j++;
            }
            res += j;
        }
        vector<int> v(r-l+1);    
        int i = l;
        j = mid+1;
        int pos = 0; 
        while(i<=mid && j<=r){
            if(nums[i]<=nums[j]) v[pos++] = ( nums[i++] );
            else v[pos++] = ( nums[j++] );
        }
        while(i<=mid) v[pos++] = ( nums[i++] );
        while(j<=r) v[pos++] = ( nums[j++] );
        pos = 0;
        for(i=l;i<=r;i++){
            nums[i] = v[pos++];
        }
        return res;   
    }
};

• 方法二

class BIT{
private:
    static const int N = 100010;
    int c[N] = {0};
    
    int lowbit(int x){
        return x&-x;
    }

public:

    void update(int x){
        while(x<N){
            c[x]++;
            x += lowbit(x);
        }
    }

    int ask(int x){
        int res = 0;
        while(x){
            res += c[x];
            x -= lowbit(x);
        }
        return res;
    }

};


class Solution {
public:
    int reversePairs(vector<int>& nums) {
        set<long> st;
        for(long x:nums){
            st.insert(x);
            st.insert(2*x);
        }

        unordered_map<long,int> f;
        // 從1開始
        int index = 1;
        for(long x:st){
            f[x] = index++;
        }

        BIT bit;
        int ans = 0;
        for(long x:nums){
            int left = f[2*x];
            ans += bit.ask(index)-bit.ask(left);
            bit.update(f[x]);
        }
        return ans;
    }
};