POJ 1716 Integer Intervals SPFA 差分约束

用数组dist[i]表示从点0到点i-1所包含的关键点的数目；
输入数据即可转化为：dist[b+1]-dist[a]>=2      =>    dist[a]-dist[b+1]<=-2
根据实际情况还有两个约束条件：
dist[i+1]-dist[i]<=1
dist[i+1]-dist[i]>=0    =>   dist[i]-dist[i]+1<=0
于是可以建图，SPFA。

最后输出dist[max]-dist[min]即可（max与min 为输入数据出现的最大点和最小点）

/*
ID: sdj22251
PROG: subset
LANG: C++
*/
#include <iostream>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <queue>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <map>
#include <string>
#include <cstring>
#include <cmath>
#include <ctime>
#define LOCA
#define MAXN 50500
#define INF 1000000000
#define eps 1e-7
using namespace std;
struct node
{
    int v, w, next;
}edge[MAXN * 10];
int head[MAXN], e, mx, mi, d[MAXN], q[120 * MAXN], vis[MAXN];
void spfa(int src)
{
    for(int i = mi; i <= mx; i++)
        d[i] = INF;
    int h = 0, t = 1;
    d[src] = 0;
    q[0] = src;
    vis[src] = 1;
    while(h < t)
    {
        int u = q[h++];
        vis[u] = 0;
        for(int i = head[u]; i != -1; i = edge[i].next)
        {
            int v = edge[i].v;
            int w = edge[i].w;
            if(d[v] > d[u] + w)
            {
                d[v] = d[u] + w;
                if(!vis[v])
                {
                    q[t++] = v;
                    vis[v] = 1;
                }
            }
        }
    }
}
void insert(const int &x, const int &y, const int &w)
{
    edge[e].v = y;
    edge[e].w = w;
    edge[e].next = head[x];
    head[x] = e++;
}
int main()
{
    int n, u, v;
    scanf("%d", &n);
    memset(head, -1, sizeof(head));
    for(int i = 1; i <= n; i++)
    {
        scanf("%d%d", &u, &v);
        if(v + 1 > mx) mx = v + 1;
        if(mi > u) mi = u;
        insert(v + 1, u, -2);
        //insert(u, v + 1, v - u + 1);
    }
    for(int i = 0; i < mx; i++)
    {
        insert(i, i + 1, 1);
        insert(i + 1, i, 0);
    }
    spfa(mx);
    printf("%d\n", d[mx] - d[mi]);
    return 0;
}