[hdu4911]逆序对相关

思路：由于只能交换相邻的数，所以每次最多减小1个逆序对（且如果存在逆序对那么肯定可以减小1个）！于是乎。。就是统计逆序对的裸题了。树状数组或归并都行。

 #pragma comment(linker, "/STACK:10240000,10240000")

 #include <iostream>
 #include <cstdio>
 #include <algorithm>
 #include <cstdlib>
 #include <cstring>
 #include <map>
 #include <queue>
 #include <deque>
 #include <cmath>
 #include <vector>
 #include <ctime>
 #include <cctype>
 #include <set>

 using namespace std;

 #define mem0(a) memset(a, 0, sizeof(a))
 #define lson l, m, rt << 1
 #define rson m + 1, r, rt << 1 | 1
 #define define_m int m = (l + r) >> 1
 #define Rep(a, b) for(int a = 0; a < b; a++)
 #define lowbit(x) ((x) & (-(x)))
 #define constructInt4(name, a, b, c, d) name(int a = 0, int b = 0, int c = 0, int d = 0): a(a), b(b), c(c), d(d) {}
 #define constructInt3(name, a, b, c) name(int a = 0, int b = 0, int c = 0): a(a), b(b), c(c) {}
 #define constructInt2(name, a, b) name(int a = 0, int b = 0): a(a), b(b) {}

 typedef double db;
 typedef long long LL;
 typedef pair<int, int> pii;
 typedef multiset<int> msi;
 typedef multiset<int>::iterator msii;
 typedef set<int> si;
 typedef set<int>::iterator sii;
 typedef vector<int> vi;

 const int dx[] = {, , -, , , , -, -};
 const int dy[] = {, -, , , -, , , -};
 const int maxn = 1e5 + ;
 const int maxm = 1e5 + ;
 const int maxv = 1e7 + ;
 const int MD = 1e9 +;
 const int INF = 1e9 + ;
 const double PI = acos(-1.0);
 const double eps = 1e-;

 int tmp[maxn], a[maxn];

 LL merge(int l, int m, int r) {
     int p = m + , t = l;
     LL res = ;
     for (int i = l; i <= m; i++) {
         while (p <= r && a[i] > a[p]) {
             tmp[t++] = a[p++];
         }
         res += p - m - ;
         tmp[t++] = a[i];
     }
     for (int i = l; i < p; i++) a[i] = tmp[i];
     return res;
 }

 LL merge_sort(int l, int r) {
     if (l == r) return ;
     define_m;
     return merge_sort(l, m) + merge_sort(m + , r) + merge(l, m, r);
 }

 int main() {
     //freopen("in.txt", "r", stdin);
     int n;
     LL k;
     while (cin >> n >> k) {
         for (int i = ; i < n; i++) {
             scanf("%d", a + i);
         }
         LL ans = merge_sort(, n - ) - k;
         ans = max(ans, 0LL);
         cout << ans << endl;
     }
     return ;
 }

Hint:做这题的时候发现一个不容易发现的坑（与这题本身无关），多个函数相加并不一定按从左至右的顺序执行（可能是为了某种意义上地优化代码）。此代码用c++交好像w会a掉，不信可以一试~(c++貌似优化程度比较高？)