[www.luogu.org] P1972 HH's necklace *Solution*

酱油 posted @ Tue, 16 Feb 2016 21:08:37 +0800 in OI life with tags solution , 2657 readers

A problem suitable for BIT, the Binary Indexed Tree (Estimated Difficulty: above Senior, NOIP)

"Sky Gate":http://www.luogu.org/problem/show?pid=1972#

Given N, N integers and M queries on the number of different numbers between interval [l_i,r_i], answer each query.
Input: 
N
A_1 A_2 ... A_N
M
l_1 r_1
l_2 r_2
...
l_M r_M 
Output:
M lines in total , answering M queries in order with an integer every line
Sample Input:
6
1 2 3 4 3 5
3
1 2
3 5
2 6
Sample Output:
2
2
4
Explanations:
There are two distinct numbers (1 and 2) in interval [1,2]; two distinct numbers (3 and 4) in interval [3,5]; and four distinct numbers (2, 3, 4 and 5)in interval [2,6]

For all 5 testcases: 1<=N<=50000, 0<=A_i<=1000000 (1<=i<=N), 1<=M<=200000, 1<=l_i<=r_i<=N (1<=i<=M), all numbers mentioned are non-negative integers

First of all, it is very important to sort all N numbers, and order them with 1 to N positive integers representing their value. (We call it "discretization")

Disconnect all the queries first and sort by their intervals.

Use a BIT, and a linking chart pointing to the next position with the same number.

Only through GDB could you realize how interesting the code could be...

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<ctime>
#include<algorithm>
#include<cctype>
#include<string>
#include<vector>
#define hk 60100
#define lb(x) ((x)&(-(x)))
using namespace std;
inline void getint(int&x){
    char c;for(c=getchar();c>57||c<48;c=getchar());
    for(x=0;c<58&&c>47;c=getchar())x=x*10+c-48;
}typedef struct pt{int dig,ord;};
typedef struct qu{int l,r,o,ans;};
typedef struct BIT{
    int n,c[hk];void add(int x,int d){while(x<=n){c[x]+=d;x+=lb(x);}}
    int que(int x){int r=0;while(x){r+=c[x];x-=lb(x);}return r;}
};int n,i,b[hk],m,j,nxt[hk],p[hk];pt a[hk];qu q[hk<<2];BIT bit;
bool jsy(pt x,pt y){return x.dig<y.dig||x.dig==y.dig&&x.ord<y.ord;}
bool hgs(pt x,pt y){return x.ord<y.ord||x.ord==y.ord&&x.dig<y.dig;}
bool ckh(qu x,qu y){return x.l<y.l||x.l==y.l&&x.r<y.r||x.l==y.l&&x.r==y.r&&x.o<y.o;}
bool ccb(qu x,qu y){return x.o<y.o;}
int main(){
    getint(n);for(i=1;i<=n;i++){getint(a[i].dig);a[i].ord=i;}
    sort(a+1,a+n+1,jsy);b[1]=1;for(i=2;i<=n;i++)b[i]=b[i-1]+(a[i].dig>a[i-1].dig?1:0);
    for(i=1;i<=n;i++)a[i].dig=b[i];sort(a+1,a+n+1,hgs);bit.n=n;
    for(i=n;i>0;i--){nxt[i]=p[a[i].dig];p[a[i].dig]=i;}
    for(i=1;i<=n;i++)if(p[i])bit.add(p[i],1);
    getint(m);for(i=1;i<=m;i++){getint(q[i].l);getint(q[i].r);q[i].o=i;}
    sort(q+1,q+m+1,ckh);j=1;for(i=1;i<=m;i++){
        while(j<q[i].l){if(nxt[j])bit.add(nxt[j],1);j++;}
        q[i].ans=bit.que(q[i].r)-bit.que(q[i].l-1);
    }sort(q+1,q+m+1,ccb);for(i=1;i<=m;i++)printf("%d\n",q[i].ans);return 0;
}

 

I used BIT for the first time, having learnt a lot.

However there is a special solution called "Mo Dui", which could also solve it.

Waiting for someone's explanation because I'm a WEAK DISH...

 

If you have better solution, please show your solution through comments, and I'll take it seriously.

(Comments are welcomed and it is recommended that you reply in English.)

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