给定以下重定向列表
[
{
"old": "a",
"target": "b"
},
{
"old": "b",
"target": "c"
},
{
"old": "c",
"target": "d"
},
{
"old": "d",
"target": "a"
},
{
"old": "o",
"target": "n"
},
{
"old": "n",
"target": "b"
},
{
"old": "j",
"target": "x"
},
{
"old": "whatever",
"target": "something"
}
]
在这里我们可以看到第一个项目“a”应该重定向到“b”。 如果我们按照列表进行操作,我们可以看到以下模式:
a -> b
b -> c
c -> d
d -> a
所以我们最终会得到一个循环引用,因为“a”最终会指向“d”,而“d”会指向“a”。
查找循环引用的最有效方法是什么?
我在 C# 中提出了以下算法
var items = JsonConvert.DeserializeObject<IEnumerable<Item>>(json)
.GroupBy(x => x.Old)
.Select(x => x.First())
.ToDictionary(x => x.Old, x => x.Target);
var circulars = new Dictionary<string, string>();
foreach (var item in items)
{
var target = item.Value;
while (items.ContainsKey(target))
{
target = items[target];
if (target.Equals(item.Key))
{
circulars.Add(target, item.Value);
break;
}
}
}
这将给我一个包含 4 个项目的列表,如下所示:
[
{
"old": "a",
"target": "b"
},
{
"old": "b",
"target": "c"
},
{
"old": "c",
"target": "d"
},
{
"old": "d",
"target": "a"
}
]
但我只对告诉用户类似的事情感兴趣
“嘿,你不能那样做,这将是一个循环引用,因为“a”指向“b”,“b”指向“c”,“c”指向“d”,“d”指向“a”强>
那么,你们有什么建议吗? 我确定存在其他一些(更好的)算法来执行此操作...:)
最佳答案
虽然通用图循环查找算法会起作用,但由于“旧的是唯一的,目标不是” 约束,您的情况有点特殊。它实际上意味着,每个节点只能有一个后继节点,因此它最多只能是一个循环的一部分。另外,DFS-Traversing节点时,不会有任何 fork ,因此迭代DFS实现变得非常容易。
给定任意起始节点,此函数可以找到从起始节点可达的环:
/// <summary>
/// Returns a node that is part of a cycle or null if no cycle is found
/// </summary>
static string FindCycleHelper(string start, Dictionary<string, string> successors, HashSet<string> stackVisited)
{
string current = start;
while (current != null)
{
if (stackVisited.Contains(current))
{
// this node is part of a cycle
return current;
}
stackVisited.Add(current);
successors.TryGetValue(current, out current);
}
return null;
}
为了保持效率,可以扩展到到达一个已经检查过的节点时提前返回(使用previouslyVisited
):
/// <summary>
/// Returns a node that is part of a cycle or null if no cycle is found
/// </summary>
static string FindCycleHelper(string start, Dictionary<string, string> successors, HashSet<string> stackVisited, HashSet<string> previouslyVisited)
{
string current = start;
while (current != null)
{
if (previouslyVisited.Contains(current))
{
return null;
}
if (stackVisited.Contains(current))
{
// this node is part of a cycle
return current;
}
stackVisited.Add(current);
successors.TryGetValue(current, out current);
}
return null;
}
下面的函数用来维护访问集的一致性
static string FindCycle(string start, Dictionary<string, string> successors, HashSet<string> globalVisited)
{
HashSet<string> stackVisited = new HashSet<string>();
var result = FindCycleHelper(start, successors, stackVisited, globalVisited);
// update collection of previously processed nodes
globalVisited.UnionWith(stackVisited);
return result;
}
为每个old
节点调用它以检查循环。当检测到循环起始节点时,可以单独创建循环信息:
// static testdata - can be obtained from JSON for real code
IEnumerable<Item> items = new Item[]
{
new Item{ Old = "a", Target = "b" },
new Item{ Old = "b", Target = "c" },
new Item{ Old = "c", Target = "d" },
new Item{ Old = "d", Target = "a" },
new Item{ Old = "j", Target = "x" },
new Item{ Old = "w", Target = "s" },
};
var successors = items.ToDictionary(x => x.Old, x => x.Target);
var visited = new HashSet<string>();
List<List<string>> cycles = new List<List<string>>();
foreach (var item in items)
{
string cycleStart = FindCycle(item.Old, successors, visited);
if (cycleStart != null)
{
// cycle found, get detail information about involved nodes
List<string> cycle = GetCycleMembers(cycleStart, successors);
cycles.Add(cycle);
}
}
以您想要的任何方式输出您找到的循环。例如
foreach (var cycle in cycles)
{
Console.WriteLine("Cycle:");
Console.WriteLine(string.Join(" # ", cycle));
Console.WriteLine();
}
GetCycleMembers
的实现非常简单 - 它取决于正确的起始节点:
/// <summary>
/// Returns the list of nodes that are involved in a cycle
/// </summary>
/// <param name="cycleStart">This is required to belong to a cycle, otherwise an exception will be thrown</param>
/// <param name="successors"></param>
/// <returns></returns>
private static List<string> GetCycleMembers(string cycleStart, Dictionary<string, string> successors)
{
var visited = new HashSet<string>();
var members = new List<string>();
var current = cycleStart;
while (!visited.Contains(current))
{
members.Add(current);
visited.Add(current);
current = successors[current];
}
return members;
}
关于c# - 在列表中查找循环引用的最有效方法,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/46484725/