poj 1201 差分约束+spfa

非常经典的差分约束系统的建模。求最小值需要转化为求最长路。

#include <algorithm>
#include <iostream>
#include <cstring>
#include <cstdio>
using namespace std;

const int INF = 99999999;
const int N = 50002;
const int M = 200000;
int head[N];
int dist[N];
bool visit[N];
int n, m, e;
int maxn, minn;

struct Edge
{
    int v, next, w;
} edge[M];

void addEdge( int u, int v, int w )
{
    edge[e].v = v;
    edge[e].w = w;
    edge[e].next = head[u];
    head[u] = e++;
}

void spfa( int s )
{
    for ( int i = minn; i <= maxn; i++ )
    {
        dist[i] = -INF;
        visit[i] = false;
    }
    dist[s] = 0;
    visit[s] = true;
    int q[N], top = 0;
    q[top++] = s;
    while ( top )
    {
        int u = q[--top];
        visit[u] = false;
        for ( int j = head[u]; j != -1; j = edge[j].next )
        {
            int v = edge[j].v, w = edge[j].w;
            if ( dist[v] < dist[u] + w )
            {
                dist[v] = dist[u] + w;
                if ( !visit[v] )
                {
                    q[top++] = v;
                    visit[v] = true;
                }
            }
        }
    }
}

int main ()
{
    while ( scanf("%d", &n) != EOF )
    {
        e = 0;
        memset( head, -1, sizeof(head) );
        minn = INF, maxn = -INF;
        for ( int i = 1; i <= n; i++ )
        {
            int a, b, c;
            scanf("%d%d%d", &a, &b, &c);
            addEdge( a, b + 1, c );
            maxn = max( maxn, b + 1 );
            minn = min( minn, a );
        }
        for ( int i = minn + 1; i <= maxn; i++ )
        {
            addEdge( i, i - 1, -1 );
            addEdge( i - 1, i, 0 );
        }
        spfa(minn);
        printf("%d
", dist[maxn]);
    }
    return 0;
}