Merge sort implementation JavaScript

Question

I was trying to implement merge sort algorithm in JavaScript without built in methods like slice(), splice(), etc. It doesn't work exactly I wish. Can you help me to figure out where is bug hidden? Getting output [3, 5, 5, 3, 7] instead of [3, 5, 5, 7, 8].

 // Merge Sort implementation // sort implementation function sort(arr, start, mid, end) { // Creation and filling the temp arrays let lArray = []; let rArray = []; for (let i = 0; i <= mid - start; i++) { lArray[i] = arr[start + i]; } for (let j = 0; j <= end - mid - 1; j++) { rArray[j] = arr[mid + 1 + j]; } // Sorting and updating current array let i = 0; let j = 0; let k = start; while (i < lArray.length && j < rArray.length) { if (lArray[i] < rArray[j]) { arr[k] = lArray[i]; i++; k++; } else { arr[k] = rArray[j]; j++; k++; } } // Handling last element in lArray or rArray i < lArray.length ? arr[k] = lArray[i] : arr[k] = rArray[j]; } // Recursive Merge Sort function recursiveMergeSort(arr, start, end) { if (start < end) { let mid = Math.floor(((end) + start) / 2); //console.log(start, end, mid); recursiveMergeSort(arr, start, mid); recursiveMergeSort(arr, mid + 1, end); sort(arr, start, mid, end); } } function mergeSort(arr) { let start = 0; let end = arr.length - 1; recursiveMergeSort(arr, start, end); return (arr) } console.log(mergeSort([5, 8, 3, 7, 5]));

Answer 1

// Handling last element in lArray or rArray is not correct.

After merging arrays you have to check the rests and copy untreated tail if it does exist

 while (i < lArray.length) {
     arr[k] = lArray[i];
     i++;
     k++;
 }
 while (j < rArray.length) {
     arr[k] = rArray[j];
     j++;
     k++;
 }

Replacing ternary operator by this code gives correct results for given data

Answer 2

Example of a somewhat optimized version of top down merge sort. It does a one time allocation of a working array and uses a pair of mutually recursive functions to change the direction of merge for each level of recursion. Takes about 1/4 second to sort 1 million pseudo-random integers on my system:

 // merge sort top down function merge(a, b, bgn, mid, end) { var i = bgn // left: a[bgn,mid) var j = mid // right: a[mid,end) var k = bgn // index for b[] while(true){ if(a[i] <= a[j]){ // if left <= right b[k++] = a[i++] // copy left if(i < mid) // if not end of left continue // continue back to while do // else copy rest of right b[k++] = a[j++] while(j < end) break // and break } else { // else left > right b[k++] = a[j++] // copy right if(j < end) // if not end of right continue // continue back to while do // else copy rest of left b[k++] = a[i++] while(i < mid) break // and break } } } function sortatob(a, b, bgn, end) { // sort a to b if ((end-bgn) < 2){ b[bgn] = a[bgn] return } var mid = Math.floor(bgn + (end - bgn) / 2) sortatoa(a, b, bgn, mid) sortatoa(a, b, mid, end) merge(a, b, bgn, mid, end) } function sortatoa(a, b, bgn, end) { // sort a to a if ((end-bgn) < 2) return var mid = Math.floor(bgn + (end - bgn) / 2) sortatob(a, b, bgn, mid) sortatob(a, b, mid, end) merge(b, a, bgn, mid, end) } function mergesort(a) { // entry function if(a.length < 2) return var b = new Array(a.length) // allocate temp array sortatoa(a, b, 0, a.length) // start with sort a to a } var a = new Array(1000000) for (i = 0; i < a.length; i++) { a[i] = parseInt(Math.random() * 1000000000) } console.time('measure') mergesort(a) console.timeEnd('measure') for (i = 1; i < a.length; i++) { if(a[i-1] > a[i]){ console.log('error') break } }