每日一题——Ultra-QuickSort

题目

Ultra-QuickSort

题解

这道题本质是求逆序对的个数，用归并思想来做。

参考代码

#include<iostream>
using namespace std;
typedef long long LL;
const int N = 1e5 + 10;
int n;
int a[N],tmp[N];
LL merge_sort(int l, int r){if(l == r) return 0;int mid = l + r >> 1;LL ans = merge_sort(l, mid) + merge_sort(mid + 1, r);int i = l, j = mid + 1, k = 0;while(i <= mid && j <= r){if(a[i] <= a[j]) tmp[k ++] = a[i ++];else{tmp[k ++] = a[j ++];ans += mid - i + 1;}}while(i <= mid) tmp[k ++] = a[i ++];while(j <= r) tmp[k ++] = a[j ++];for(int i = l, j = 0; i <= r; i ++, j ++) a[i] = tmp[j];return ans;
}
int main(){while(cin >> n, n){for(int i = 1; i <= n; i ++) cin >> a[i];cout << merge_sort(1,n) << endl;}return 0;
}