POJ-1716 Integer Intervals

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题目大意：

给你n个区间[a, b]，问你是否存在一个集合Z，使得|Z∩[a,b]| >= 2

解题思路：

同poj 1201，简化版

POJ-1201 Intervals

代码：

#include <queue>#include <cstdio>#include <vector>using namespace std;const int maxn = 10000 + 5;const int INF = 0x3f3f3f3f;typedef struct node{    int to, w;    node(int a = 0, int b = 0){        to = a; w = b;    }}Edge;vector<Edge> vec[maxn];int dis[maxn], vis[maxn];int spfa(int n){    Edge v;    int len, p;    queue<int> q;    while(!q.empty()) q.pop();    for(int i = 0; i <= n; ++i){        vis[i] = 0;        dis[i] = INF;    }    dis[0] = 0;    vis[0] = 1;    q.push(0);    while(!q.empty()){        p = q.front(); q.pop();        len = vec[p].size();        for(int i = 0; i < len; ++i){            v = vec[p][i];            if(dis[v.to] > dis[p] + v.w){                dis[v.to] = dis[p] + v.w;                if(!vis[v.to]){                    vis[v.to] = 1;                    q.push(v.to);                }            }        }        vis[p] = 0;    }    return -dis[n];}int main(){    int n, a, b;    while(~scanf("%d", &n)){        int e = 0;        for(int i = 0; i < maxn; ++i) vec[i].clear();        for(int i = 0; i < n; ++i){            scanf("%d%d", &a, &b);            e = (e < b ? b : e);            vec[a].push_back(Edge(b+1, -2));        }        for(int i = 0; i <= e; ++i){            vec[i].push_back(Edge(i+1, 0));            vec[i+1].push_back(Edge(i, 1));        }        printf("%d\n", spfa(e + 1));    }    return 0;}

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