1≤n≤500,1≤k≤3 1 \leq n \leq 500, 1 \leq k \leq 31≤n≤500,1≤k≤3
我们把每个区间的端点作为网络流图中的顶点,那么每个区间的长度便是端点
所建边的费用,容量为1。
建图:
1:端点离散化。
2:源点连点1,容量为k费用为0。
3:最后一个点连汇点,容量为k费用为0。
4:第i个点连第i+1个点,容量为INF费用为0,。
5:连接每个区间的端点,容量为1,费用为长度。
求最小费用最大流。
#include<stdio.h>#include<algorithm>#include<string.h>#include<queue>using namespace std;#define swap(a,b){int c;c=a,a=b,b=c;}const int maxm = 1000005;const int INF = 1e9 + 7;struct node{int u, v, flow, cost, next;}edge[maxm];int dis[maxm], head[maxm], cur[maxm], pre[maxm];int f[maxm], map[maxm], l[maxm], r[maxm], a[maxm];int s, t, n, m, cnt;void init(){cnt = 0, s = 0, t = m + 1;memset(head, -1, sizeof(head));}void add(int u, int v, int w, int cost){edge[cnt].u = u, edge[cnt].v = v;edge[cnt].flow = w, edge[cnt].cost = cost;edge[cnt].next = head[u], head[u] = cnt++;edge[cnt].u = v, edge[cnt].v = u;edge[cnt].flow = 0, edge[cnt].cost = -cost;edge[cnt].next = head[v], head[v] = cnt++;}int bfs(){queue<int>q;for (int i = 0;i <= 1000001;i++) dis[i] = INF;memset(pre, -1, sizeof(pre));dis[s] = 0;q.push(s);int rev = 0;while (!q.empty()){int u = q.front();q.pop();for (int i = head[u];i != -1;i = edge[i].next){int v = edge[i].v;if (dis[v] > dis[u] + edge[i].cost&&edge[i].flow){dis[v] = dis[u] + edge[i].cost;pre[v] = i;q.push(v);}}}if (dis[t] == INF) return 0;return 1;}int MCMF(){int minflow, ans = 0;while (bfs()){minflow = INF;for (int i = pre[t];i != -1;i = pre[edge[i].u])minflow = min(minflow, edge[i].flow);for (int i = pre[t];i != -1;i = pre[edge[i].u]){edge[i].flow -= minflow;edge[i ^ 1].flow += minflow;}ans += minflow*dis[t];}return ans;}int main(){int i, j, k, sum, id = 0;scanf("%d%d", &n, &k);for (i = 1;i <= n;i++){scanf("%d%d", &l[i], &r[i]);if (l[i] > r[i]) swap(l[i], r[i]);f[++id] = l[i], f[++id] = r[i];}sort(f + 1, f + 1 + id);m = 0;a[++m] = m;map[f[1]] = m;for (i = 2;i <= id;i++){if (f[i] != f[i - 1]) a[++m] = m;map[f[i]] = m;}init();add(s, 1, k, 0);add(m, t, k, 0);for (i = 1;i < m;i++)add(i, i + 1, INF, 0);for (i = 1;i <= n;i++)add(map[l[i]], map[r[i]], 1, -(r[i] - l[i]));printf("%d\n", -MCMF());return 0;}