题目链接：
http://www.lydsy.com/JudgeOnline/problem.php?id=2763
题解：
d[x][kk]表示从s到x用了kk次免费机会的最少花费。
代码：
#include<iostream>
#include<cstdio>
#include<algorithm>
#include<queue>
#include<vector>
#include<cstring>
#define mp make_pair
#define X first
#define Y second
using namespace std;const int maxn = 10000;int n, m, k;
vector<pair<int, int> > G[maxn];int d[maxn][22];
bool inq[maxn][22];
void spfa(int s) {memset(d, 0x7f, sizeof(d));memset(inq, 0, sizeof(inq));queue<pair<int,int> > Q;d[s][0] = 0, inq[s][0] = 1, Q.push(mp(s,0));while (!Q.empty()) {int u = Q.front().X,kk=Q.front().Y;Q.pop();inq[u][kk] = 0;for (int i = 0; i < G[u].size(); i++) {int v = G[u][i].X, w = G[u][i].Y;if (d[v][kk]>d[u][kk] + w) {d[v][kk] = d[u][kk] + w;if (!inq[v][kk]) {inq[v][kk] = 1, Q.push(mp(v, kk));}}if (kk + 1 <= k&&d[v][kk + 1] > d[u][kk]) {d[v][kk + 1] = d[u][kk];if (!inq[v][kk + 1]) {inq[v][kk + 1] = 1, Q.push(mp(v, kk + 1));}}}}
}void init() {for (int i = 0; i < n; i++) G[i].clear();
}int main() {while (scanf("%d%d%d", &n, &m, &k) == 3) {init();int s, t;scanf("%d%d", &s, &t);for (int i = 0; i < m; i++) {int u, v, w;scanf("%d%d%d", &u, &v, &w);G[u].push_back(mp(v, w));G[v].push_back(mp(u, w));}spfa(s);printf("%d\n", d[t][k]);}return 0;
}
