TypeError: Cannot read property 'left' of undefined

• I am trying to solve the graph datastructure problem of hacker rank.
• My method should return the minimal cost of providing libraries to all.
• Providing my hacker rank question here https://www.hackerrank.com/challenges/torque-and-development/problem?h_l=interview&playlist_slugs%5B%5D%5B%5D=interview-preparation-kit&playlist_slugs%5B%5D%5B%5D=graphs&isFullScreen=true
• I am planning to use BFS and solve the problem.
• so I read some articles and found this link https://medium.com/basecs/breaking-down-breadth-first-search-cebe696709d9
• but the problem is I am getting this error TypeError: Cannot read property 'left' of undefined
• I used the medium code.
• providing my code snippet below.
• can you tell me how to fix it.

function roadsAndLibraries(n, c_lib, c_road, cities) {
    console.log("roadsAndLibraries n--->", n);
    console.log("roadsAndLibraries c_lib--->", c_lib);
    console.log("roadsAndLibraries c_road--->", c_road);
    console.log("roadsAndLibraries cities--->", cities);
    var m = new Map();
    m.set('a', 2);
    m.set('b', 3);
    m.set('b', 3);
    m.set('b', 2);
    m.set('b', 1);

    console.log("map value--->", m);
        // Check that a root node exists.

    // if (rootNode === null) {
    //     return;
    // }

    // Check that a root node exists.
    if (n === null) {
        console.log("n root node--->", n);
        return;
    }

    // Create our queue and push our root node into it.
    // var queue = [];
    // queue.push(rootNode);

    // Create our queue and push our root node into it.
    var queue = [];
    queue.push(n);

    console.log(" queue.push--->", queue);


    while (queue.length > 0) {
        // Create a reference to currentNode, at the top of the queue.
        var currentNode = queue[0];

        // If currentNode has a left child node, add it to the queue.
        if (currentNode.left !== null) {
            queue.push(currentNode.left)
        }
        // If currentNode has a right child node, add it to the queue.
        if (currentNode.right !== null) {
            queue.push(currentNode.right)
        }
        // Remove the currentNode from the queue.
        queue.shift()
    }




}

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