离散化 + 线段树

#include <cstdio>
#include <cstring>
#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;
#define maxn 200000 + 10
#define lson L, mid, rt<<1
#define rson mid, R, rt<<1|1

int X1[maxn], X2[maxn];
int n;
int lazy[maxn<<2];
int mark[maxn];

int compress(int *x1, int *x2)
{
    vector<int> x;

    for(int i=0; i<n; i++)
    {
        x.push_back(x1[i]);
        x.push_back(x2[i]);
    }

    sort(x.begin(), x.end());
    x.erase(unique(x.begin(), x.end()), x.end());

    for(int i=0; i<n; i++)
    {
        x1[i] = find(x.begin(), x.end(), x1[i]) - x.begin();
        x2[i] = find(x.begin(), x.end(), x2[i]) - x.begin();
    }

    return x.size() - 1;
}

void pushdown(int L, int R, int rt)
{
    lazy[rt<<1] = lazy[rt];
    lazy[rt<<1|1] = lazy[rt];
    lazy[rt] = 0;
}

void update(int l, int r, int v, int L, int R, int rt)
{
    if(l==L && r==R)
    {
        lazy[rt] = v;
        return ;
    }

    int mid = (L + R) >> 1;
    if(lazy[rt]) pushdown(L, R, rt);

    if(r <= mid) update(l, r, v, lson);
    else if(l >= mid) update(l, r, v, rson);
    else
    {
        update(l, mid, v, lson);
        update(mid, r, v, rson);
    }
}

void traverse(int L, int R, int rt)
{
    if(L+1 == R)
    {
        mark[L] = lazy[rt];
        return ;
    }

    int mid = (L + R) >> 1;
    if(lazy[rt]) pushdown(L, R, rt);
    traverse(lson);
    traverse(rson);
}

void solve()
{
    int w = compress(X1, X2);
    int num = 0;
    for(int i=0; i<n; i++)
    {
        update(X1[i], X2[i], ++num, 0, w, 1);
    }
    traverse(0, w, 1);

    bool vis[maxn];
    memset(vis, 0, sizeof(vis));
    int ans = 0;
    for(int i=0; i<=w; i++)
      if(mark[i] && !vis[mark[i]])
        {
            vis[mark[i]] = 1;
            ans++;
        }
    printf("%d\n", ans);
}

int main()
{
    int l;
    while(~scanf("%d%d", &n, &l))
    {
        memset(lazy, 0, sizeof(lazy));
        memset(mark, 0, sizeof(mark));

        for(int i=0; i<n; i++)
            scanf("%d%d", &X1[i], &X2[i]);

        solve();
    }
    return 0;
}