【Codeforces 459D】Pashmak and Parmida's problem
【链接】 我是链接,点我呀:)
【题意】
g(i)表示a[i..n]中等于a[i]的数字的个数
让你求出来(i,j) 这里i<j
的二元组个数
且f(i)>g(j)
【题解】
求出来两个数组g[N]和f[N]; (用map就行) 要算出(i< j)且f[i]>g[j]的个数 我们可以先把 g[1..n]全都加入到树状数组中。 然后顺序枚举i 遇到i就把g[i]从树状数组中删掉. 这样就只包括g[i+1..n]这些数字了 则,我们求一个1..f[i]-1的前缀和就好了 这样就是左端点为i满足题意的j的个数了【代码】
import java.io.*;
import java.util.*;
public class Main {
static InputReader in;
static PrintWriter out;
public static void main(String[] args) throws IOException{
//InputStream ins = new FileInputStream("E:\\rush.txt");
InputStream ins = System.in;
in = new InputReader(ins);
out = new PrintWriter(System.out);
//code start from here
new Task().solve(in, out);
out.close();
}
static int N = (int)1e6;
static class BIT{
int a[];
BIT(){
a = new int[N+10];
}
public int lowbit(int x) {
return x&(-x);
}
public void Add(int x,int y) {
while (x<=N) {
a[x] = a[x]+y;
x = x + lowbit(x);
}
}
public long sum(int x) {
long temp = 0;
while (x>=1) {
temp = temp + a[x];
x = x - lowbit(x);
}
return temp;
}
}
static class Task{
int n;
int a[],b[],c[];
HashMap<Integer,Integer> pre,aft;
public void solve(InputReader in,PrintWriter out) {
a = new int[N+10];b = new int[N+10];c = new int[N+10];
n = in.nextInt();
pre = new HashMap<Integer,Integer>();
aft = new HashMap<Integer,Integer>();
for (int i = 1;i <= n;i++) a[i] = in.nextInt();
for (int i = 1;i <= n;i++) {
int cnt;
if (pre.containsKey(a[i])) {
cnt = pre.get(a[i]);
cnt++;
}else {
cnt = 1;
}
pre.put(a[i], cnt);
b[i] = cnt;
}
for (int i = n;i >= 1;i--) {
int cnt;
if (aft.containsKey(a[i])) {
cnt = aft.get(a[i]);
cnt++;
}else {
cnt = 1;
}
aft.put(a[i], cnt);
c[i] = cnt;
}
BIT mybit = new BIT();
for (int i = 1;i <= n;i++) mybit.Add(c[i], 1);
long ans = 0;
for (int i = 1;i <= n;i++) {
mybit.Add(c[i], -1);
ans = ans + mybit.sum(b[i]-1);
}
out.println(ans);
}
}
static class InputReader{
public BufferedReader br;
public StringTokenizer tokenizer;
public InputReader(InputStream ins) {
br = new BufferedReader(new InputStreamReader(ins));
tokenizer = null;
}
public String next(){
while (tokenizer==null || !tokenizer.hasMoreTokens()) {
try {
tokenizer = new StringTokenizer(br.readLine());
}catch(IOException e) {
throw new RuntimeException(e);
}
}
return tokenizer.nextToken();
}
public int nextInt() {
return Integer.parseInt(next());
}
}
}