题目链接：https://vjudge.net/problem/POJ-2299
题目大意
归并排序求逆序数经典题。
分析
略。
代码如下
1 #include <cmath>
2 #include <ctime>
3 #include <iostream>
4 #include <string>
5 #include <vector>
6 #include <cstdio>
7 #include <cstdlib>
8 #include <cstring>
9 #include <queue>
10 #include <map>
11 #include <set>
12 #include <algorithm>
13 #include <cctype>
14 #include <stack>
15 #include <deque>
16 #include <list>
17 #include <sstream>
18 #include <cassert>
19 using namespace std;
20
21 #define INIT() ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
22 #define Rep(i,n) for (int i = 0; i < (int)(n); ++i)
23 #define For(i,s,t) for (int i = (int)(s); i <= (int)(t); ++i)
24 #define rFor(i,t,s) for (int i = (int)(t); i >= (int)(s); --i)
25 #define ForLL(i, s, t) for (LL i = LL(s); i <= LL(t); ++i)
26 #define rForLL(i, t, s) for (LL i = LL(t); i >= LL(s); --i)
27 #define foreach(i,c) for (__typeof(c.begin()) i = c.begin(); i != c.end(); ++i)
28 #define rforeach(i,c) for (__typeof(c.rbegin()) i = c.rbegin(); i != c.rend(); ++i)
29
30 #define pr(x) cout << #x << " = " << x << " "
31 #define prln(x) cout << #x << " = " << x << endl
32
33 #define LOWBIT(x) ((x)&(-x))
34
35 #define ALL(x) x.begin(),x.end()
36 #define INS(x) inserter(x,x.begin())
37 #define UNIQUE(x) x.erase(unique(x.begin(), x.end()), x.end())
38 #define REMOVE(x, c) x.erase(remove(x.begin(), x.end(), c), x.end()); // 删去 x 中所有 c
39 #define TOLOWER(x) transform(x.begin(), x.end(), x.begin(),::tolower);
40 #define TOUPPER(x) transform(x.begin(), x.end(), x.begin(),::toupper);
41
42 #define ms0(a) memset(a,0,sizeof(a))
43 #define msI(a) memset(a,0x3f,sizeof(a))
44 #define msM(a) memset(a,-1,sizeof(a))
45
46 #define MP make_pair
47 #define PB push_back
48 #define ft first
49 #define sd second
50
51 template<typename T1, typename T2>
52 istream &operator>>(istream &in, pair<T1, T2> &p) {
53 in >> p.first >> p.second;
54 return in;
55 }
56
57 template<typename T>
58 istream &operator>>(istream &in, vector<T> &v) {
59 for (auto &x: v)
60 in >> x;
61 return in;
62 }
63
64 template<typename T>
65 ostream &operator<<(ostream &out, vector<T> &v) {
66 Rep(i, v.size()) out << v[i] << " \n"[i == v.size()];
67 return out;
68 }
69
70 template<typename T1, typename T2>
71 ostream &operator<<(ostream &out, const std::pair<T1, T2> &p) {
72 out << "[" << p.first << ", " << p.second << "]" << "\n";
73 return out;
74 }
75
76 inline int gc(){
77 static const int BUF = 1e7;
78 static char buf[BUF], *bg = buf + BUF, *ed = bg;
79
80 if(bg == ed) fread(bg = buf, 1, BUF, stdin);
81 return *bg++;
82 }
83
84 inline int ri(){
85 int x = 0, f = 1, c = gc();
86 for(; c<48||c>57; f = c=='-'?-1:f, c=gc());
87 for(; c>47&&c<58; x = x*10 + c - 48, c=gc());
88 return x*f;
89 }
90
91 template<class T>
92 inline string toString(T x) {
93 ostringstream sout;
94 sout << x;
95 return sout.str();
96 }
97
98 inline int toInt(string s) {
99 int v;
100 istringstream sin(s);
101 sin >> v;
102 return v;
103 }
104
105 //min <= aim <= max
106 template<typename T>
107 inline bool BETWEEN(const T aim, const T min, const T max) {
108 return min <= aim && aim <= max;
109 }
110
111 typedef long long LL;
112 typedef unsigned long long uLL;
113 typedef vector< int > VI;
114 typedef vector< bool > VB;
115 typedef vector< char > VC;
116 typedef vector< double > VD;
117 typedef vector< string > VS;
118 typedef vector< LL > VL;
119 typedef vector< VI > VVI;
120 typedef vector< VB > VVB;
121 typedef vector< VS > VVS;
122 typedef vector< VL > VVL;
123 typedef vector< VVI > VVVI;
124 typedef vector< VVL > VVVL;
125 typedef pair< int, int > PII;
126 typedef pair< LL, LL > PLL;
127 typedef pair< int, string > PIS;
128 typedef pair< string, int > PSI;
129 typedef pair< string, string > PSS;
130 typedef pair< double, double > PDD;
131 typedef vector< PII > VPII;
132 typedef vector< PLL > VPLL;
133 typedef vector< VPII > VVPII;
134 typedef vector< VPLL > VVPLL;
135 typedef vector< VS > VVS;
136 typedef map< int, int > MII;
137 //typedef unordered_map< int, int > uMII;
138 typedef map< LL, LL > MLL;
139 typedef map< string, int > MSI;
140 typedef map< int, string > MIS;
141 typedef set< int > SI;
142 typedef stack< int > SKI;
143 typedef queue< int > QI;
144 typedef priority_queue< int > PQIMax;
145 typedef priority_queue< int, VI, greater< int > > PQIMin;
146 const double EPS = 1e-8;
147 const LL inf = 0x7fffffff;
148 const LL infLL = 0x7fffffffffffffffLL;
149 const LL mod = 1e9 + 7;
150 const int maxN = 5e5 + 7;
151 const LL ONE = 1;
152 const LL evenBits = 0xaaaaaaaaaaaaaaaa;
153 const LL oddBits = 0x5555555555555555;
154
155 int a[maxN], b[maxN], N;
156 LL ans;
157
158 void Merge(int l, int mid, int r) {
159 int i = l, j = mid + 1, k = l;
160 while(i <= mid && j <= r) {
161 if(a[i] <= a[j]) b[k++] = a[i++];
162 else {
163 b[k++] = a[j++];
164 ans += (LL)(mid - i + 1);
165 }
166 }
167 while(i <= mid) b[k++] = a[i++];
168 while(j <= r) b[k++] = a[j++];
169 For(h, l, r) a[h] = b[h];
170 }
171
172 void MergeSort(int l, int r) {
173 if(l >= r) return;
174 int mid = (l + r) >> 1;
175
176 MergeSort(l, mid);
177 MergeSort(mid + 1, r);
178 Merge(l, mid, r);
179 }
180
181 int main(){
182 //freopen("MyOutput.txt","w",stdout);
183 //freopen("input.txt","r",stdin);
184 //INIT();
185 while(scanf("%d", &N) && N) {
186 ans = 0;
187 Rep(i, N) scanf("%d", &a[i]);
188 MergeSort(0, N - 1);
189 printf("%lld\n", ans);
190 }
191 return 0;
192 }