## Tuesday, July 21, 2015

### LeetCode OJ - Rectangle Area

Problem:

Solution:

This is a tricky problem, if not properly think through, it is really easy to fall trap into many of the cases that might arise.

The key idea of solving this problem is that

1. Two line segments, if intersect at all, intersect as a line segment.
2. We can compute the x-intersection and y-intersection separately.
3. A line sweep can be used to compute the 1 dimensional line segment intersection problem.
Without using the sort in the intersection routine, the cases can grow like crazy. 4! = 24 cases. The sorting is what make things easy!

Code:

#include "stdafx.h"

// https://leetcode.com/problems/rectangle-area/

#include "LEET_RECTANGLE_AREA.h"
#include <map>
#include <iostream>
#include <sstream>
#include <vector>
#include <string>
#include <algorithm>

using namespace std;

namespace _LEET_RECTANGLE_AREA
{
class Solution
{
private:
// Do a simple line sweep
int intersection_length(int p1, int p2, int q1, int q2)
{
// Step 1: Sorting
int coord[4] = { p1, p2, q1, q2 };
int event[4] = { 1, 1, 2, 2};
for (int i = 0; i < 4; i++)
{
for (int j = i + 1; j < 4; j++)
{
if (coord[j] < coord[i])
{
swap(coord[j], coord[i]);
swap(event[j], event[i]);
}
}
}
bool inOne = false;
bool inTwo = false;
bool hasIntersection = false;
int start = 0;
int end = 0;
for (int i = 0; i < 4; i++)
{
if (event[i] == 1) { inOne = !inOne; }
if (event[i] == 2) { inTwo = !inTwo; }
if (inOne && inTwo)
{
hasIntersection = true;
start = coord[i];
}
else if (hasIntersection)
{
end = coord[i];
break;
}
}
if (hasIntersection)
{
return end - start;
}
else
{
return 0;
}
}

public:
int computeArea(int A, int B, int C, int D, int E, int F, int G, int H)
{
int x_intersection = this->intersection_length(A, C, E, G);
int y_intersection = this->intersection_length(B, D, F, H);
int rect1_size = (C - A) * (D - B);
int rect2_size = (G - E) * (H - F);
int intersect_size = x_intersection * y_intersection;

return rect1_size + rect2_size - intersect_size;
}
};
};

using namespace _LEET_RECTANGLE_AREA;

int LEET_RECTANGLE_AREA()
{
Solution solution;
cout << solution.computeArea(-3,0,3,4,0,-1,9,2) << endl;
return 0;
}