Dijkstra算法
Algorithm copied from Wikipedia.org: http://en.wikipedia.org/wiki/Dijkstra%27s_algorithm#Algorithm
==================================================================================================
* Let the node at which we are starting be called the initial node. Let the distance of node Y be the distance from the initial
* node to Y. Dijkstra's algorithm will assign some initial distance values and will try to improve them step by step.
* 1. Assign to every node a distance value: set it to zero for our initial node and to infinity for all other nodes.
* 2. Mark all nodes as unvisited. Set initial node as current.
* 3. For current node, consider all its unvisited neighbors and calculate their tentative distance. For example, if current node
* (A) has distance of 6, and an edge connecting it with another node (B) is 2, the distance to B through A will be 6+2=8. If
* this distance is less than the previously recorded distance, overwrite the distance.
* 4. When we are done considering all neighbors of the current node, mark it as visited. A visited node will not be checked
* ever again; its distance recorded now is final and minimal.
* 5. If all nodes have been visited, finish. Otherwise, set the unvisited node with the smallest distance (from the initial node,
* considering all nodes in graph) as the next "current node" and continue from step 3
==================================================================================================
* Let the node at which we are starting be called the initial node. Let the distance of node Y be the distance from the initial
* node to Y. Dijkstra's algorithm will assign some initial distance values and will try to improve them step by step.
* 1. Assign to every node a distance value: set it to zero for our initial node and to infinity for all other nodes.
* 2. Mark all nodes as unvisited. Set initial node as current.
* 3. For current node, consider all its unvisited neighbors and calculate their tentative distance. For example, if current node
* (A) has distance of 6, and an edge connecting it with another node (B) is 2, the distance to B through A will be 6+2=8. If
* this distance is less than the previously recorded distance, overwrite the distance.
* 4. When we are done considering all neighbors of the current node, mark it as visited. A visited node will not be checked
* ever again; its distance recorded now is final and minimal.
* 5. If all nodes have been visited, finish. Otherwise, set the unvisited node with the smallest distance (from the initial node,
* considering all nodes in graph) as the next "current node" and continue from step 3
有一个更牛的图：
