看了一下全是线段树。
维护一个堆，在每张海报开始和结束的地方查看顶部是哪张海报标记下来
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAXN 10005
#define BEGIN 0
#define DUMMY 1
#define END 2

struct Event
{
    int pos, order, eventType;
};

int heap[MAXN], heapEnd;

int Compare(const void *a, const void *b)
{
    Event *eventA = (Event*)a, *eventB = (Event*)b;
    // Order by pos first in inc order
    if (eventA->pos != eventB->pos) return eventA->pos - eventB->pos;
    else
    {
        // Then order by event type BEGIN first and then END
        if (eventA->eventType != eventB->eventType) return eventA->eventType - eventB->eventType;
        // Then order by order in desc order for BEGIN; or inc order for END
        if (eventA->eventType == BEGIN) return eventB->order - eventA->order;
        else return eventA->order - eventB->order;
    }
}

void Push(int order)
{
    heap[++heapEnd] = order;
    int i = heapEnd;
    while (i > 1)
    {
        if (heap[i >> 1] > heap[i]) return;
        int tmp = heap[i >> 1];
        heap[i >> 1] = heap[i];
        heap[i] = tmp;
        i >>= 1;
    }
}

void Pop()
{
    heap[1] = heap[heapEnd--];
    int i = 1;
    while ((i << 1) <= heapEnd)
    {
        int l = i;
        if (heap[i << 1] > heap[l]) l = (i << 1);
        if (heap[(i << 1) + 1] > heap[l]) l = (i << 1) + 1;
        if (l == i) return;
        int tmp = heap[l];
        heap[l] = heap[i];
        heap[i] = tmp;
        i = l;
    }
}

int main()
{
    int i, caseCount, n, l, r, ans;
    char visible[MAXN], offWall[MAXN];
    Event events[MAXN * 3];
    scanf("%d", &caseCount);
    while (caseCount--)
    {
        memset(visible, 0, sizeof(visible));
        memset(offWall, 0, sizeof(offWall));
        scanf("%d", &n);
        for (i = 0; i < n; i++)
        {
            scanf("%d %d", &l, &r);
            int idx = i * 3;
            events[idx].pos = l, events[idx].order = i, events[idx].eventType = BEGIN;
            idx++;
            events[idx].pos = r, events[idx].order = i, events[idx].eventType = END;
            // Need to check what's on top at r + 1
            // Consider the case:
            /*
3
1 5
1 2
4 5
            */
            // 0 1 2 3 4
            // - - - - -
            // - -
            //       - -
            idx++;
            events[idx].pos = r + 1, events[idx].eventType = DUMMY;
        }
        n *= 3;
        qsort(events, n, sizeof(Event), Compare);
        heapEnd = 0;
        for (i = 0; i < n; i++)
        {
            // Beginning of a poster, add to heap
            if (events[i].eventType == BEGIN)
            {
                Push(events[i].order);
            }
            // Mark topmost as visible
            if (heapEnd > 0) visible[heap[1]] = 1;
            if (events[i].eventType == END)
            {
                // Mark the poster as off-wall
                offWall[events[i].order] = 1;
                // Pop heap if the topmost poster is already off-wall
                while (heapEnd > 0 && offWall[heap[1]])
                {
                    Pop();
                }
            }
        }
        n /= 3;
        for (ans = 0, i = 0; i < n; i++) ans += visible[i];
        printf("%d\n", ans);
    }
    return 0;
}