poj 1716 Integer Intervals

与 zoj 1508 Intervals相同，均为差分约束，http://blog.csdn.net/unimen/article/details/6719738

/*
  Name: poj 1716Integer Intervals
  Author: Unimen
  Date: 27/08/11 09:53
  Description: 差分约束 
*/

/*
解题报告：与zoj 1508Intervals 相同 
 
*/ 

#include <iostream>
#include <cstring>
using namespace std;
const int MAXN = 10010;

struct 
{
	int a, b;
}Edge[MAXN];
int n;
int iMax, iMin;
int dis[MAXN];

int bellman_ford()
{
	int i;
	bool bUpdata = true;
	while(bUpdata)
	{
		bUpdata = false;
		for(i=1; i<=n; ++i)
		{
			if(dis[Edge[i].b] < dis[Edge[i].a] + 2)
			{
				dis[Edge[i].b] = dis[Edge[i].a] + 2;
				bUpdata = true;
			}
		}
		
		for(i=iMin; i<=iMax; ++i)
		{
			if(dis[i] < dis[i-1])
			{
				dis[i] = dis[i-1];
				bUpdata = true;
			}
		}
		
		for(i=iMax; i>=iMin; --i)
		{
			if(dis[i-1] < dis[i] -1)
			{
				dis[i-1] = dis[i] - 1;
				bUpdata = true;
			}
		}
	}
	return dis[iMax] - dis[iMin-1];
}

int main()
{
	int i;
	while(cin>>n)
	{
		iMax = 0, iMin = 1000000000;
		for(i=1; i<=n; ++i)
		{
			int a, b;
			cin>>a>>b;
			Edge[i].a = a - 1;
			Edge[i].b = b;
			if(iMin > a)
				iMin = a;
			if(iMax < b)
				iMax = b;
		}
		
		memset(dis, 0, sizeof(dis));
		cout<<bellman_ford()<<endl;
	}
	return 0;
}