如何在JavaScript元素的层次结构中检测循环 [英] How to detect a loop in a hierarchy of javascript elements

问题描述

我有一个元素列表，每个元素都有一个ID和一个父ID.我想做的是检测此层次结构"中是否存在循环，并显示哪个ID开始循环.

I have a list of elements, each has an ID and a parent ID. What I want to do is detect when there is a loop in this 'hierarchy', and show which ID starts the loop.

list = [
  {
    id: '1',
    parent: '2'
  },
  {
    id: '2',
    parent: '3'
  },
  {
    id: '3',
    parent: '4'
  },
    {
    //This id is causing the loop
    id: '4',
    parent: '1'
  }
]

我已经尝试过构建树，该树在没有循环的情况下可以使用，但是在没有循环的情况下可以使用:

I have tried this to build the tree, which works when there's no loop, but does not work with a loop:

function treeify(list, idAttr, parentAttr, childrenAttr) {
    if (!idAttr) idAttr = 'id';
    if (!parentAttr) parentAttr = 'parent';
    if (!childrenAttr) childrenAttr = 'children';
    var treeList = [];
    var lookup = {};
    list.forEach(function(obj) {
        lookup[obj[idAttr]] = obj;
        obj[childrenAttr] = [];
    });
    list.forEach(function(obj) {
        if (obj[parentAttr] != null) {
            lookup[obj[parentAttr]][childrenAttr].push(obj);
        } else {
            treeList.push(obj);
        }
    });
    return treeList;
};

我也无法检测到何时出现循环.

I also cannot detect when there is a loop.

我想返回导致循环的元素的ID，以便我修复其背后的数据.

I'd like to return the ID of the element that caused the loop to allow me to fix the data behind it.

推荐答案

您可以应用白色-灰色-黑色着色来检测在访问其后代时被(重新)访问的节点(我已将您的图形简化为亲子对):

You can apply white-grey-black coloring to detect nodes that are (re)visited while visiting their descendants (I've simplified your graph to a list of parent-child pairs):

graph = [
    [2, 1],
    [3, 2],
    [1300023, 3],
    [1, 1300023],
];


colors = {}


function visit(vertex) {
    if (colors[vertex] === 'black') {
        // black = ok
        return; 
    }

    if (colors[vertex] === 'grey') {
        // grey = visited while its children are being visited
        // cycle!
        console.log('cycle', colors); 
        return; 
    }

    // mark as being visited
    colors[vertex] = 'grey';

    // visit children
    graph.forEach(edge => {
        if (edge[0] === vertex)
            visit(edge[1]);
    });

    // mark as visited and ok
    colors[vertex] = 'black'

}

visit(1)

此方法的一个很好的例子: https://algorithms.tutorialhorizo​​n.com/graph-detect-cycle-in-a-directed-graph-using-colors/

A nice illustration of this approach: https://algorithms.tutorialhorizon.com/graph-detect-cycle-in-a-directed-graph-using-colors/

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