You want to processe a sequence of n distinct integers by swapping two adjacent sequence elements until the sequence is sorted in ascending order. Then how many times it need.
For example, 1 2 3 5 4, we only need one operation : swap 5 and 4.
Input
The input consists of a number of test cases. Each case consists of two lines: the first line contains a positive integer n (n <= 1000); the next line contains a permutation of the n integers from 1 to n.
Output
For each case, output the minimum times need to sort it in ascending order on a single line.
Sample Input
3 1 2 3 4 4 3 2 1
Sample Output
0 6
题解:
#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
using namespace std;
const int maxn = 1005;
int n, ans;
int a[maxn], b[maxn];
void Merge_Sort(int l, int r){
if(l < r){
int mid = (l+r) / 2;
int i = l, j = mid+1, id = l;
Merge_Sort(l, mid);
Merge_Sort(mid+1, r);
while(i <= mid && j <= r){
if(a[i] <= a[j]) b[id++] = a[i++];
else b[id++] = a[j++], ans += mid - i + 1;//a[i~mid]都比a[j]要打,所以产生mid-i+1个逆序对
}
while(i <= mid) b[id++] = a[i++];
while(j <= r) b[id++] = a[j++];
for(int k = l; k <= r; k++) a[k] = b[k];
}
}
int main()
{
while(~scanf("%d", &n)){
ans = 0;
memset(b, 0, sizeof b);
memset(a, 0, sizeof a);
for(int i = 1; i <= n; i++)
scanf("%d", &a[i]);
Merge_Sort(1, n);
printf("%d\n", ans);
}
return 0;
}