Check if there are any nodes that point back to already visited nodes

graphCycles.js

var detectGraphCycles = function(graph, rootId) {
    if (typeof graph[rootId] === 'undefined') { return false };
    var currentNode = graph[rootId];
    var visited = {};
    var graphHasCycles = false;
    while (currentNode && currentNode.next) {
        if (typeof currentNode.id !== 'undefined') {
            if (visited[currentNode.id] === true) {
                currentNode = false;
                graphHasCycles = true;
            } else {
                visited[currentNode.id] = true;
                currentNode = graph[currentNode.next];
            }
        } else {
            // so that we stop here
            currentNode = false;
        }
    }
    return graphHasCycles;
}




var testCases = [
    {},
    {
        1: {
            id: 1,
            next: 3
        },
        2: {
            id: 2,
            next: 3
        },
        3: {
            id: 3,
            next: 1
        },
    },
    {
        1: {
            id: 1,
            next: 2
        },
        2: {
            id: 2,
            next: 3
        },
        3: {
            id: 3,
            next: null
        },
    },
    {
        1: {
            id: 1,
            next: 2
        },
        2: {
            id: 2,
            next: 3
        },
        3: {
            id: 3,
            next: 1
        },
    }
]

for (var test of testCases) {
    let result = detectGraphCycles(test, 1);
    console.log(test);
    if (result) {
        console.log('This graph has cycles');
    } else {
        console.log('no cycles');
    }
    console.log('');
}