|
1 |
| -#Floyd–Warshall algorithm |
| 1 | +#Floyd–Warshall Algorithm |
| 2 | + |
| 3 | +In computer science, the**Floyd–Warshall algorithm** is an algorithm for finding |
| 4 | +shortest paths in a weighted graph with positive or negative edge weights (but |
| 5 | +with no negative cycles). A single execution of the algorithm will find the |
| 6 | +lengths (summed weights) of shortest paths between all pairs of vertices. Although |
| 7 | +it does not return details of the paths themselves, it is possible to reconstruct |
| 8 | +the paths with simple modifications to the algorithm. |
| 9 | + |
| 10 | +##Algorithm |
| 11 | + |
| 12 | +The Floyd–Warshall algorithm compares all possible paths through the graph between |
| 13 | +each pair of vertices. It is able to do this with`O(|V|^3)` comparisons in a graph. |
| 14 | +This is remarkable considering that there may be up to`|V|^2` edges in the graph, |
| 15 | +and every combination of edges is tested. It does so by incrementally improving an |
| 16 | +estimate on the shortest path between two vertices, until the estimate is optimal. |
| 17 | + |
| 18 | +Consider a graph`G` with vertices`V` numbered`1` through`N`. Further consider |
| 19 | +a function`shortestPath(i, j, k)` that returns the shortest possible path |
| 20 | +from`i` to`j` using vertices only from the set`{1, 2, ..., k}` as |
| 21 | +intermediate points along the way. Now, given this function, our goal is to |
| 22 | +find the shortest path from each`i` to each`j` using only vertices |
| 23 | +in`{1, 2, ..., N}`. |
| 24 | + |
| 25 | + |
| 26 | + |
| 27 | + |
| 28 | + |
| 29 | + |
| 30 | +This formula is the heart of the Floyd–Warshall algorithm. |
| 31 | + |
| 32 | +##Example |
| 33 | + |
| 34 | +The algorithm above is executed on the graph on the left below: |
| 35 | + |
| 36 | + |
| 37 | + |
| 38 | +In the tables below`i` is row numbers and`j` is column numbers. |
| 39 | + |
| 40 | + |
| 41 | +**k = 0** |
| 42 | + |
| 43 | +|| 1| 2| 3| 4| |
| 44 | +|:-----:|:---:|:---:|:---:|:---:| |
| 45 | +|**1**|0|∞|−2| ∞| |
| 46 | +|**2**|4|0|3| ∞| |
| 47 | +|**3**|∞|∞|0| 2| |
| 48 | +|**4**|∞|−1| ∞| 0| |
| 49 | + |
| 50 | + |
| 51 | +**k = 1** |
| 52 | + |
| 53 | +|| 1| 2| 3| 4| |
| 54 | +|:-----:|:---:|:---:|:---:|:---:| |
| 55 | +|**1**| 0| ∞| −2| ∞| |
| 56 | +|**2**| 4| 0| 2| ∞| |
| 57 | +|**3**| ∞| ∞| 0| 2| |
| 58 | +|**4**| ∞| −| ∞| 0| |
| 59 | + |
| 60 | + |
| 61 | +**k = 2** |
| 62 | + |
| 63 | +|| 1| 2| 3| 4| |
| 64 | +|:-----:|:---:|:---:|:---:|:---:| |
| 65 | +|**1**|0|∞|−2| ∞| |
| 66 | +|**2**|4|0| 2| ∞| |
| 67 | +|**3**|∞|∞| 0| 2| |
| 68 | +|**4**|3|−1| 1| 0| |
| 69 | + |
| 70 | + |
| 71 | +**k = 3** |
| 72 | + |
| 73 | +|| 1| 2| 3| 4| |
| 74 | +|:-----:|:---:|:---:|:---:|:---:| |
| 75 | +|**1**|0|∞|−2| 0| |
| 76 | +|**2**|4|0|2| 4| |
| 77 | +|**3**|∞|∞|0| 2| |
| 78 | +|**4**|3|−1| 1| 0| |
| 79 | + |
| 80 | + |
| 81 | +**k = 4** |
| 82 | + |
| 83 | +|| 1| 2| 3| 4| |
| 84 | +|:-----:|:---:|:---:|:---:|:---:| |
| 85 | +|**1**|0|−1| −2| 0| |
| 86 | +|**2**|4|0| 2| 4| |
| 87 | +|**3**|5|1| 0| 2| |
| 88 | +|**4**|3|−1| 1| 0| |
2 | 89 |
|
3 | 90 | ##References
|
4 | 91 |
|
5 | 92 | -[Wikipedia](https://en.wikipedia.org/wiki/Floyd%E2%80%93Warshall_algorithm)
|
| 93 | +-[YouTube (by Abdul Bari)](https://www.youtube.com/watch?v=oNI0rf2P9gE&list=PLLXdhg_r2hKA7DPDsunoDZ-Z769jWn4R8&index=74) |
| 94 | +-[YouTube (by Tushar Roy)](https://www.youtube.com/watch?v=LwJdNfdLF9s&list=PLLXdhg_r2hKA7DPDsunoDZ-Z769jWn4R8&index=75) |