A RRR-TL Traversal is an hierarchical algorithm that gives each sub-tree equal priority among its siblings with respect to its parent recursively

RecursiveRoundRobinTreeLeafTraversal.js

// ====================================================================
// RECURSIVE ROUND ROBIN (RRR) TREE LEAF TRAVERSAL ALGORITHM (JS ES6+)
// METHOD IDEA: Cagil Seker [ÇAĞIL ŞEKER] (MadChuckle) - [email protected]
// SAMPLE ALGORITHM: Apass.Jack (https://cs.stackexchange.com/users/91753/apass-jack)
// ====================================================================
// This is a gist for my (MadChuckle) question at StackOverflow
// Link: 'https://cs.stackexchange.com/questions/95951/is-there-a-name-and-efficient-algorithm-for-this-tree-traversal-method'
// 
// This is based on the the pseudo code (and later an implementation) from Apass.Jack
//
// *****************************************************************************
// * A RRR-TL Traversal is an hierarchical algorithm that gives each sub-tree  *
// * equal priority among its siblings with respect to its parent recursively  *
// * Refer to the linked SO question above for more information.               *
// *****************************************************************************
// 
// INPUT: [[['A', 'B', ['C']], ['E']], 'D', ['F', [['G', 'H'], [['I', 'J']], 'K']]]
// OUTPUT: ["A","D","F","E","G","B","I","C","K","H","J"]

let myTree = [[['A', 'B', ['C']], ['E']], 'D', ['F', [['G', 'H'], [['I', 'J']], 'K']]]

let result = rrrTraversal(myTree)

console.log(JSON.stringify(result))

// procedure rrrTraversal(rootedTree):
function rrrTraversal(t) {
    if (!Array.isArray(t)) {
        return [t]
    }
    
    // Merge a parent with its child as a minor performance improvement.
    if (t.length == 1) {
        return rrrTraversal(t[0])
    }

    let ll = [] // ll := an empty list of lists

    // foreach subtree s of rootedTree from left to right:
    t.forEach (s => {
        let ss = rrrTraversal(s)
        ll.push(ss) //push ss to the back of ll
    })

    return zip(ll)
}

// procedure zip(listOflists):
function zip(ll) {
    let zipped = []

    // while listOflists is not empty:
    while (ll.length) {
        let toRemove = [] // indices to be removed
        
        // foreach list li in listOflists from front to end:
        for (let i = 0; i < ll.length; i++) {
            let li = ll[i]
            
            if (!li.length) { // if li is empty:
                toRemove.unshift(i) // add index to the beginning of toRemove
            } else {
                let fi = li.splice(0, 1)[0] // remove the first item from li and hold it
                zipped.push(fi) // push that first item to the back of the zipped
            }
        }
        
        toRemove.forEach(index => ll.splice(index, 1))
    }

    return zipped
}