Problem:
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Union of Rectangles

Given a set of $N$ axis-aligned rectangles in the plane, find the area of regions which are covered by at least one rectangle.

Input

The input is given in the following format.

$N$

$x1_1$ $y1_1$ $x2_1$ $y2_1$

$x1_2$ $y1_2$ $x2_2$ $y2_2$

:

$x1_N$ $y1_N$ $x2_N$ $y2_N$

($x1_i, y1_i$) and ($x2_i, y2_i$) are the coordinates of the top-left corner and the bottom-right corner of the $i$-th rectangle respectively.

Constraints

$ 1 \leq N \leq 2000 $

$ −10^9 \leq x1_i < x2_i\leq 10^9 $

$ −10^9 \leq y1_i < y2_i\leq 10^9 $

Output

Print the area of the regions.

Sample Input 1

2
0 0 3 4
1 2 4 3

Sample Output 1

13

Sample Input 2

3
1 1 2 5
2 1 5 2
1 2 2 5

Sample Output 2

7

Sample Input 3

4
0 0 3 1
0 0 1 3
0 2 3 3
2 0 3 3

Sample Output 3

8

Solution in C (judge verdict: Accepted)

// AOJ DSL_4_A Union of Rectangles
// 2018.5.14 bal4u

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#if 1
#define gc() getchar_unlocked()
#else
#define gc() getchar()
#endif
int in()
{
	int n = 0, c = gc();
	if (c == '-') {	c = gc();
		do n = 10*n + (c & 0xf), c = gc(); while (c >= '0');
		return -n;
	}
	do n = 10*n + (c & 0xf), c = gc(); while (c >= '0');
	return n;
}

#define MAX_N 2005
#define MAX   4005

int x1[MAX_N], y1[MAX_N], x2[MAX_N], y2[MAX_N];
int x[MAX], xs;
int y[MAX], ys;
char s[MAX][MAX];

int cmp(const void *a, const void *b) { return *(int *)a - *(int *)b; }

int xbsch(int a)
{
	int m, l = 0, r = xs;

    while (l < r) {
        m = (l + r) >> 1;
        if (x[m] < a) l = m + 1; else r = m;
    }
	return l;
}
int ybsch(int a)
{
	int m, l = 0, r = ys;

    while (l < r) {
        m = (l + r) >> 1;
        if (y[m] < a) l = m + 1; else r = m;
    }
	return l;
}

int uniq(int *a, int n)
{
	int i, j;
	
	for (i = 0, j = 1; j < n; j++) {
		while (j < n && a[j] == a[i]) j++;
		if (j < n) if (++i != j) a[i] = a[j];
	}
	return i+1;
}

int main()
{
	int N, i, r, c, r1, c1, r2, c2;
	long long ans;

	N = in(); xs = ys = 0;
	for (i = 0; i < N; i++) {
		x1[i] = c = in(), y1[i] = r = in();
		x[xs++] = c, y[ys++] = r;
		x2[i] = c = in(), y2[i] = r = in();
		x[xs++] = c, y[ys++] = r;
	}
	qsort(x, xs, sizeof(int), cmp); xs = uniq(x, xs);
	qsort(y, ys, sizeof(int), cmp); ys = uniq(y, ys);

	for (i = 0; i < N; i++) {
		c1 = xbsch(x1[i]), c2 = xbsch(x2[i]);
		r1 = ybsch(y1[i]), r2 = ybsch(y2[i]);
		for (r = r1; r < r2; r++) for (c = c1; c < c2; c++) s[r][c] = 1;
	}

	ans = 0;
	for (r = 0; r < ys; r++) for (c = 0; c < xs; c++) {
		if (s[r][c]) {
			c1 = c++; while (s[r][c]) c++;
			ans += (long long)(x[c]-x[c1])*(y[r+1]-y[r]);
		}
	}
	printf("%lld\n", ans);
	return 0;
}