Merge sort

mergeSort

function mergeSort(array, start, end){
    
    if (start === end) return [array[start]];

    var mid = Math.floor( (start + end) / 2 );
    
    var arr1 = mergeSort(array, start, mid);
    var arr2 = mergeSort(array, mid+1, end);
    var result = merge(arr1, arr2);
    
    return result;
}

function merge(arr1, arr2){
    var result = [];
    var ptr1 = 0;
    var ptr2 = 0;
    
    while( (ptr1 < arr1.length) && (ptr2 < arr2.length) ){
        
        if (arr1[ptr1] < arr2[ptr2]){
            result.push(arr1[ptr1]);
            ptr1++;
        } else if (arr1[ptr1] > arr2[ptr2]){
            result.push(arr2[ptr2]);
            ptr2++;
        } else {
            result.push(arr1[ptr1]);
            result.push(arr2[ptr2]);
            ptr1++;
            ptr2++;
        }
    }
    
    console.log(arr1, arr2);
    result = result.concat(arr1.slice(ptr1).concat(arr2.slice(ptr2)));
    console.log("result:", result);
    return result;
}

// var array = [10,9,8,7,6,5,4,3,2,1];
// console.log(mergeSort(array, 0, array.length-1));