「模版」归并排序求逆序对

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NOIP考前练一练模版

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#include <iostream>
#include <cstdio>
#include <cctype>
#define DEBUG(x) std::cerr << #x << "=" << x << std::endl
inline int read() {
    char ch = getchar(); int res = 0; int flag = 1;
    for(; !isdigit(ch); ch = getchar()) if(ch == '-') flag = -1;
    for(; isdigit(ch); ch = getchar()) res = (res << 3) + (res << 1) + ch - '0';
    return (flag * res);
}
const int N_MAX = 100007;
int n, a[N_MAX], b[N_MAX];
long long ans;
void merge_sort(int l, int r) {
	if(l == r) return;
	int mid = (l + r) / 2;
	merge_sort(l, mid), merge_sort(mid + 1, r);
	int i = l; int j = mid + 1;
	for(int k = l; k <= r; k ++) {
		if(j > r || i <= mid && a[i] < a[j]) b[k] = a[i ++];
		else b[k] = a[j ++], ans += mid - i + 1;
	}
	for(int k = l; k <= r; k ++) a[k] = b[k];
}
int main() {
	for(int i = 1; i < 100007; i ++) a[i] = i;
	for(int i = 1; i <= n; i ++) {
		int x = read(); int y = read();
		std::swap(a[x], a[y]);
	}
	
	merge_sort(1, N_MAX - 1);
	printf("%lld\n", ans);
	return 0;
}