逆序对

洛谷-P1908-逆序对

分析

见《进阶指南》第37页。

实现

#include <iostream>
using namespace std;

typedef long long LL;
const int N = 500010;
int n, a[N], temp[N];

LL merge_sort (int l, int r) {
    if (l == r) return 0;
    
    int mid = l + r >> 1;
    LL res = merge_sort(l, mid) + merge_sort(mid + 1, r);

    int i = l, j = mid + 1, k = l;
    while (i <= mid && j <= r) {
        if (a[i] <= a[j]) {
            temp[k ++] = a[i ++];
        } else {
            temp[k ++] = a[j ++];
            res += mid - i + 1;
        }
    }

    while (i <= mid) temp[k ++] = a[i ++];
    while (j <= r) temp[k ++] = a[j ++];
    for (int i = l; i <= r; ++ i) a[i] = temp[i];
    return res;
}

int main () {
    scanf("%d", &n);
    for (int i = 1; i <= n; ++ i) scanf("%d", &a[i]);
    printf("%lld", merge_sort(1, n));
    return 0;
}