# Dijkstra's algorithm ¶

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If $G$ is a weighted graph with only positives weights, then you can use Dijkstra's algorithm (complexity $O(n^2)$).

Dijkstra will give you the shortest path starting from a vertex (A for instance) to any other vertices (B, ...).

The easiest way to use Dijkstra's algorithm is a table of the steps (i=0 to n) by the vertices (A, B, ...). A case like (0,A) is representing the shortest path value after 0 iterations.

At $i=0$, the shortest path value is $0$ for your starting vertex and $+\infty$ for all other vertices. You will also consider your selected vertex done, meaning that the value as the shortest path value is the final result.

At $i+1$, (it's a bit hard to explain, learn with the example)

• if there is an edge between the last done vertex and another one having the weight $w$
• if the other one is not "done"
• and the other one weight is greater $w+\text{last done value}$
• then we replace the shortest path value by $w+\text{last done value}$

Then, once you did all that, you will set as done the vertex having the least weight until all of them are done.

## Example ¶ Step A B C D E Done
$0$ $0$ $+\infty$ $+\infty$ $+\infty$ $+\infty$ $A$

$A$ is linked to $B$ by 10 and to $E$ by 5. We are replacing both $+\infty$ since the new weights are smaller ($0+10$ and $0+5$). We are writing $A(10)$ to remember what value was used for B sum ($0+10$, the $0$ is A value).

Step A B C D E Done
$0$ $0$ $+\infty$ $+\infty$ $+\infty$ $+\infty$ $A$
$1$ $0$ $A(10)$ $+\infty$ $+\infty$ $A(5)$ $E$

For the third line, $E(8)$ in $B$ column means that we added the value of $E=5$ plus the weight for $weight(E \to B)=3$. The idea is the same, so we are filling our table.

Step A B C D E Done
$0$ $0$ $+\infty$ $+\infty$ $+\infty$ $+\infty$ $A$
$1$ $A(10)$ $+\infty$ $+\infty$ $A(5)$ $E$
$2$ $E(8)$ $E(14)$ $E(7)$ $D$
$3$ $E(8)$ $D(13)$ $B$
$4$ $B(9)$ $C$

For the interpretation, the shortest path from A to ... is ...

• A to B is $[A-E-B]$ (weight=8)
• A to C is $[A-E-B-C]$ (weight=9)
• A to D is $[A-E-D]$ (weight=7)
• A to D is $[A-E]$ (weight=5)

If you are wondering how I got these results, for $A to B$, the weight is the value in the column: $8$ and for the path

• B
• go back to E (since $E(8)$)
• go back to A (since $A(5)$)
• giving us $[A-E-B]$