poj 2528 Line segment tree interval merge

Description

The citizens of Bytetown, AB, could not stand that the candidates in the mayoral election campaign have been placing their electoral posters at all places at their whim. The city council has finally decided to build an electoral wall for placing the posters and introduce the following rules:

Every candidate can place exactly one poster on the wall.
All posters are of the same height equal to the height of the wall; the width of a poster can be any integer number of bytes (byte is the unit of length in Bytetown).
The wall is divided into segments and the width of each segment is one byte.
Each poster must completely cover a contiguous number of wall segments.

They have built a wall 10000000 bytes long (such that there is enough place for all candidates). When the electoral campaign was restarted, the candidates were placing their posters on the wall and their posters differed widely in width. Moreover, the candidates started placing their posters on wall segments already occupied by other posters. Everyone in Bytetown was curious whose posters will be visible (entirely or in part) on the last day before elections.
Your task is to find the number of visible posters when all the posters are placed given the information about posters' size, their place and order of placement on the electoral wall.

Input

The first line of input contains a number c giving the number of cases that follow. The first line of data for a single case contains number 1 <= n <= 10000. The subsequent n lines describe the posters in the order in which they were placed. The i-th line among the n lines contains two integer numbers l _i and ri which are the number of the wall segment occupied by the left end and the right end of the i-th poster, respectively. We know that for each 1 <= i <= n, 1 <= l _i <= ri <= 10000000. After the i-th poster is placed, it entirely covers all wall segments numbered l _i , l _i +1 ,... , ri.

Output

For each input data set print the number of visible posters after all the posters are placed.

The picture below illustrates the case of the sample input.

Sample Input

Sample Output

Topic meaning:

It is to give you a poster. Posters will be posted in each section, and the ones posted at the back will cover the previous ones, so that you can ask how many posters you can see.

At the beginning, I thought that the interval merging of line segment trees was a problem, but this problem stuck me for a long time, because it needed to be discretized. I have been exposed to discretization before, but it is not commonly used, so I read other people's code, and I just looked at it at first. I don't understand, but I can still understand by outputting each step in the end.

The original ones are (1, 4), (2, 6), (8, 10), (3, 4), (7, 10). After discretization, these intervals are changed to (1, 4), ( 2, 5), (7, 8), (3, 4), (6, 8).

Code after a long time to complete:

#include <stdio.h>
#include <string.h>
#include <algorithm>
using namespace std;
#define lson l,mid,ri<<1
#define rson mid +1 ,r,ri<<1|1

int n,cnt;
const int maxn = 1e5+10;

struct Node{
    int l,r,n;
} a[maxn<<2];

struct node{
    int point,num;
} s[maxn<<2];

int map[maxn<<1][2],ans,vis[maxn<<1];

int cmp(node x,node y){
    return x.point<y.point;
}

void Build(int l,int r,int ri){
    a[ri].l = l;
    a[ri].r = r;
    a[ri].n = 0;
    if (l == r){
        return ;
    }
    int mid = (l+r)>>1;
    Build(lson);
    Build(rson);
}

void insert(int ri,int l,int r,int m){
    if(a[ri].l == l && a[ri].r == r){
        a[ri].n = m;
        return;
    }
    int mid = (a[ri].l+a[ri].r)>>1;
    if(a[ri].n>0){
        a[ri<<1].n = a[ri<<1|1].n = a[ri].n;
        a[ri].n = 0;
    }
    if(l>=a[ri<<1|1].l)
        insert(ri<<1|1,l,r,m);
    else if(r<=a[ri<<1].r)
        insert(ri<<1,l,r,m);
    else{
        insert(ri<<1,l,mid,m);
        insert(ri<<1|1,mid+1,r,m);
    }
}

void solve(int ri){
    if(a[ri].n){
        if(!vis[a[ri].n]){
            ans++;
            vis[a[ri].n] = 1;
        }
        return;
    }
    solve(ri<<1);
    solve(ri<<1|1);
    return;
}

int main(){
    int t;
    scanf("%d",&t);
    while(t--){
        scanf("%d",&n);
        for(int i = 0; i < n; i++){
            scanf("%d%d",&map[i][0],&map[i][1]);
            s[i<<1].point = map[i][0];
            s[i<<1|1].point = map[i][1];
            s[i<<1].num = -(i+1);
            s[i<<1|1].num = i+1;
        }
//        for (int i = 0; i < 2*n; i++){
//            printf("%d %d %d\n",i,s[i].point,s[i].num);
//        }
        sort(s,s+2*n,cmp);
//        printf("\n");
//        for (int i = 0; i < 2*n; i++){
//            printf("%d %d %d\n",i,s[i].point,s[i].num);
//        }
        int temp = s[0].point,cnt = 1;
        for(int i = 0; i < 2*n; i++){
            if(temp != s[i].point){
                cnt++;
                temp = s[i].point;
            }
            if(s[i].num < 0)
                map[-s[i].num-1][0] = cnt;
            else
                map[s[i].num-1][1] = cnt;
        }
        Build(1,cnt,1);
        for(int i = 0; i < n; i++){
//            printf("%d %d\n",map[i][0],map[i][1]);
            insert(1,map[i][0],map[i][1],i+1);
        }
        memset(vis,0,sizeof(vis));
        years = 0;
        solve(1);
        printf("%d\n",ans);
    }

    return 0;
}

Work hard, come on!

poj 2528 Line segment tree interval merge

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