Reverse Pairs

Posted 2020-10-04

For an array A, if i < j, and A [i] > A [j], called (A [i], A [j]) is a reverse pair.
return total of reverse pairs in A.

Example

Given A = [2, 4, 1, 3, 5] , (2, 1), (4, 1), (4, 3) are reverse pairs. return 3

Solution 1: the same as the leetcode question 315. Count of Smaller Numbers After Self

class Solution {
public:
    /**
     * @param A an array
     * @return total of reverse pairs
     */
    long long reversePairs(vector<int>& A) {
        // Write your code here
        long long res=0;
        vector<int> t;
        for (int i=A.size()-1;i>=0;i--){
            int index=lower_bound(t.begin(),t.end(),A[i])-t.begin();
            t.insert(t.begin()+index,A[i]);
            res+=index;
        }
        return res;
    }
};