Question

I wrote this implementation of Dijksta’s Algorithm, which at each iteration of the loop while Q is not empty instead of finding the minimum element of the queue it takes the head of the queue.

Here is the code i wrote

#include <stdio.h>
#include <limits.h>

#define INF INT_MAX
int N;
int Dist[500];
int Q[500];
int Visited[500];
int Graph[500][500];

void Dijkstra(int b){
     int H = 0;
     int T = -1;
     int j,k;

Dist[b] = 0;

Q[T+1] = b;
T = T+1;

while(T>=H){
    j = Q[H];
    Visited[j] = 1;
    for (k = 0;k < N; k++){
        if(!Visited[k] && Dist[k] > Graph[j][k] + Dist[j] && Graph[j][k] != -1){
            Dist[k] = Dist[j]+Graph[j][k];
            Q[T+1] = k;
            T = T+1;
        }
    }

    H = H+1;
}
}  

int main(){

int src,target,m;
int a,w,b,i,j;

scanf("%d%d%d%d",&N,&m,&src,&target);

for(i = 0;i < N;i ++){
    for(j = 0;j < N;j++){
        Graph[i][j] = -1;
    }
}

for(i = 0; i< N; i++){
    Dist[i] = INF;
    Visited[i] = 0;
}


for(i = 0;i < m; i++){
    scanf("%d%d%d",&a,&b,&w);
    a--;
    b--;
    Graph[a][b] = w;
    Graph[b][a] = w;
}

Dijkstra(src-1);


if(Dist[target-1] == INF){
    printf("NO");
}else {
    printf("YES\n%d",Dist[target-1]);
}

return 0;
}

I ran this for all the test cases i ever found and it gave a correct answer.
My question is the why do we need to find the min at all? Can anyone explain this to me in plain english ? Also i need a test case which proves my code wrong.

Answer 1

Take a look at this sample:

1-(6)-> 2 -(7)->3
  \          /
   (7)     (2)
     \    /
       4

I.e. you have edge with length 6 from 1 to 2, edge with length 7 from 2 to 3, edge with length 7 from 1 to 4 and edge from 4 to 3. I believe your algorithm will think shortest path from 1 to 3 has length 13 through 2, while actually best solution is with length 9 through 4.

Hope this make it clear.

EDIT: sorry this example did not brake the code. Have a look at this one:

8 9 1 3
1 5 6
5 3 2
1 2 7
2 3 2
1 4 7
4 3 1
1 7 3
7 8 2
8 3 2

Your output is Yes 8. While a path 1->7->8->3 takes only 7. Here is a link on ideone