Shortest Paths with Negative Edge Weights

Bellman-Ford

Shortest Paths

Solution

Finding Negative Cycles

Solution

Simple Linear Programming

You can also use shortest path algorithms to solve the following problem (a very simple linear program).

Given variables $x_1,x_2,\dots ,x_N$ with constraints in the form $x_i-x_j\ge c$, compute a feasible solution.

Resources

Problems

Timeline (USACO Camp):

• equivalent to Timeline (Gold) except $N,C\le 5000$ and negative values of $x$ are possible.

Floyd-Warshall

Solution - APSP

1const int MOD = 1000000007;
2const ll INF = 1e18;
3
4int n,m,q;
5ll dist[150][150], bad[150][150];
6
7void solve() {
8    F0R(i,n) F0R(j,n) dist[i][j] = INF, bad[i][j] = 0;
9    F0R(i,n) dist[i][i] = 0;
10    F0R(i,m) {

Problems

Modified Dijkstra

The Dijkstra code presented earlier will still give correct results if there are no negative cycles. However, the same running time bound no longer applies, as demonstrated by subtasks 1-6 of the following problem.

This problem forces you to analyze the inner workings of the three shortest-path algorithms we presented here. It also teaches you about how problemsetters could create hack cases!