Find shortest paths between all pairs of vertices in a graph.

Floyd-Warshall Algorithm
It is one of the easiest algorithms, and just involves simple dynamic programming.
The algorithm can be read from this wikipedia page.

#define SIZE 31
#define INF 1e8
double dis[SIZE][SIZE];
void init(int N)
{
for(k=0;k<N;k++)
for(i=0;i<N;i++)
dis[i][j]=INF;
}
void floyd_warshall(int N)
{
int i,j,k;
for(k=0;k<N;k++)
    for(i=0;i<N;i++)
        for (j=0;j<N;j++)
        dis[i][j]=min(dis[i][j],dis[i][k]+dis[k][j]);
}

int main()
{
//input size N
init(N);
//set values for dis[i][j]
floyd_warshall(N);
}