[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 }

View Code

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？)