[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。

 1 #pragma comment(linker, "/STACK:10240000,10240000")
 2
 3 #include <iostream>
 4 #include <cstdio>
 5 #include <algorithm>
 6 #include <cstdlib>
 7 #include <cstring>
 8 #include <map>
 9 #include <queue>
10 #include <deque>
11 #include <cmath>
12 #include <vector>
13 #include <ctime>
14 #include <cctype>
15 #include <set>
16
17 using namespace std;
18
19 #define mem0(a) memset(a, 0, sizeof(a))
20 #define lson l, m, rt << 1
21 #define rson m + 1, r, rt << 1 | 1
22 #define define_m int m = (l + r) >> 1
23 #define Rep(a, b) for(int a = 0; a < b; a++)
24 #define lowbit(x) ((x) & (-(x)))
25 #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) {}
26 #define constructInt3(name, a, b, c) name(int a = 0, int b = 0, int c = 0): a(a), b(b), c(c) {}
27 #define constructInt2(name, a, b) name(int a = 0, int b = 0): a(a), b(b) {}
28
29 typedef double db;
30 typedef long long LL;
31 typedef pair<int, int> pii;
32 typedef multiset<int> msi;
33 typedef multiset<int>::iterator msii;
34 typedef set<int> si;
35 typedef set<int>::iterator sii;
36 typedef vector<int> vi;
37
38 const int dx[8] = {1, 0, -1, 0, 1, 1, -1, -1};
39 const int dy[8] = {0, -1, 0, 1, -1, 1, 1, -1};
40 const int maxn = 1e5 + 7;
41 const int maxm = 1e5 + 7;
42 const int maxv = 1e7 + 7;
43 const int MD = 1e9 +7;
44 const int INF = 1e9 + 7;
45 const double PI = acos(-1.0);
46 const double eps = 1e-10;
47
48 int tmp[maxn], a[maxn];
49
50 LL merge(int l, int m, int r) {
51     int p = m + 1, t = l;
52     LL res = 0;
53     for (int i = l; i <= m; i++) {
54         while (p <= r && a[i] > a[p]) {
55             tmp[t++] = a[p++];
56         }
57         res += p - m - 1;
58         tmp[t++] = a[i];
59     }
60     for (int i = l; i < p; i++) a[i] = tmp[i];
61     return res;
62 }
63
64 LL merge_sort(int l, int r) {
65     if (l == r) return 0;
66     define_m;
67     return merge_sort(l, m) + merge_sort(m + 1, r) + merge(l, m, r);
68 }
69
70 int main() {
71     //freopen("in.txt", "r", stdin);
72     int n;
73     LL k;
74     while (cin >> n >> k) {
75         for (int i = 0; i < n; i++) {
76             scanf("%d", a + i);
77         }
78         LL ans = merge_sort(0, n - 1) - k;
79         ans = max(ans, 0LL);
80         cout << ans << endl;
81     }
82     return 0;
83 }

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？)