样例过了，看着没啥bug。

#include<bits/stdc++.h>
using namespace std;
const int N = 100010;
int n;
struct sg
{
    int x, y1, y2;
    int k;
    bool operator< (const sg &t)const
    {
        return x < t.x;
    }
}seg[N * 2];
struct Node
{
    int l, r;
    int cnt;
    int len;
}tr[N * 8];
vector<int> ys;
int find(int y)
{
    return lower_bound(ys.begin(), ys.end(), y) - ys.begin();
}
void pushup(int u)
{
    if(tr[u].cnt)
    {
        tr[u].len = ys[tr[u].r + 1] - ys[tr[u].l];
    }
    else if(tr[u].l != tr[u].r)
    {
        tr[u].len = tr[u << 1].len + tr[u << 1 | 1].len;
    }
    else tr[u].len = 0;
}
void build(int u, int l, int r)
{
    tr[u] = {l, r, 0, 0};
    if(l != r) 
    {
        int mid = (l + r) >> 1;
        build(u << 1, l, mid);
        build(u << 1 | 1, mid + 1, r);
    }
}
void change(int u, int l, int r, int k)
{
    if(tr[u].l >= l && tr[u].r <= r)
    {
        tr[u].cnt += k;
        pushup(u);
    }
    else
    {
        int mid = (tr[u].l + tr[u].r) >> 1;
        if(l <= mid) change(u << 1, l, r, k);
        if(r > mid) change(u << 1 | 1, l, r, k);
        pushup(u);
    }
}
int main()
{
    cin >> n;
    ys.clear();
    for(int i = 0, j = 0; i < n; i ++) 
    {
        int x1, y1, x2, y2;
        cin >> x1 >> y1 >> x2 >> y2;
        seg[j ++] = {x1, y1, y2, 1};
        seg[j ++] = {x2, y1, y2, -1};
        ys.push_back(y1);
        ys.push_back(y2);
    }
    sort(ys.begin(), ys.end());
    ys.erase(unique(ys.begin(), ys.end()), ys.end());
    build(1, 0, ys.size() - 2);
    sort(seg , seg + n * 2);
    int res = 0;
    for(int i = 0; i < n * 2; i ++)
    {
        if(i > 0) res += tr[1].len * (seg[i].x - seg[i - 1].x);
        change(1, find(seg[i].y1), find(seg[i].y2) - 1, seg[i].k);
    }
    printf("%d\n", res);
    return 0;
}