Inversions
Problem Description
There are N integers (1<=N<=65537) A1, A2,.. AN (0<=Ai<=10^9). You need to find amount of such pairs (i, j) that 1<=i<j<=N and A[i]>A[j].
Input
The first line of the input contains the number N. The second line contains N numbers A1…AN.
Output
Write amount of such pairs.
Sample test(s)
Input
5 2 3 1 5 4
Output
3
Solution
代码语言：javascript复制#include <bits/stdc .h>
int main() {
std::ios::sync_with_stdio(false);
int n{};
std::cin >> n;
std::vector<int> vec(n, 0);
std::vector<int> tmp(n, 0);
for(int i = 0; i < n; i ) {
std::cin >> vec[i];
}
long long res{0};
std::function<void(int, int)> mergeSort;
mergeSort = [&](int left, int right){
if(right - left > 1) {
int mid = (left (right - left) / 2);
int p = left;
int q = mid;
int index = left;
mergeSort(left, mid);
mergeSort(mid, right);
while(p < mid || q < right) {
if(q >= right || (p < mid && vec[p] <= vec[q])) {
tmp[index ] = vec[p ];
} else {
tmp[index ] = vec[q ];
res = mid - p;
}
}
for(index = left; index < right; index ) {
vec[index] = tmp[index];
}
}
};
mergeSort(0, n);
std::cout << res;
return 0;
}
