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伤情最是晚凉天，憔悴厮人不堪怜。邀酒催肠三杯醉，寻香惊梦五更寒。钗头凤斜卿有泪，荼蘼花了我无缘。小楼寂寞新雨月，也难如钩也难圆。
2022年6月7日
BFS - 广度优先算法（Breadth First Search）
BFS 常用于在 遍历 和 最短路径 的情况，BFS 代码模板如下：
/**
* Return the length of the shortest path between root and target node.
*/
int BFS(Node root, Node target) {
Queue<Node> queue; // store all nodes which are waiting to be processed
int step = 0; // number of steps neeeded from root to current node
// initialize
add root to queue;
// BFS
while (queue is not empty) {
step = step + 1;
// iterate the nodes which are already in the queue
int size = queue.size();
for (int i = 0; i < size; ++i) {
Node cur = the first node in queue;
return step if cur is target;
for (Node next : the neighbors of cur) {
add next to queue;
}
remove the first node from queue;
}
}
return -1; // there is no path from root to target
}
有时，确保我们永远 不会访问一个结点两次 很重要。否则，我们可能陷入无限循环。如果是这样，我们可以在上面的代码中添加一个 哈希集 来解决这个问题：
/**
* Return the length of the shortest path between root and target node.
*/
int BFS(Node root, Node target) {
Queue<Node> queue; // store all nodes which are waiting to be processed
Set<Node> used; // store all the used nodes
int step = 0; // number of steps neeeded from root to current node
// initialize
add root to queue;
add root to used;
// BFS
while (queue is not empty) {
step = step + 1;
// iterate the nodes which are already in the queue
int size = queue.size();
for (int i = 0; i < size; ++i) {
Node cur = the first node in queue;
return step if cur is target;
for (Node next : the neighbors of cur) {
if (next is not in used) {
add next to queue;
add next to used;
}
}
remove the first node from queue;
}
}
return -1; // there is no path from root to target
}
