So what's our total time cost? O(nlog⁡2n)O(n\log_{2}{n})O(nlog2​n). The log⁡2n\log_{2}{n}log2​n comes from the number of times we have to cut nnn in half to get down to sublists of just 1 element (our base case). The additional nnn comes from the time cost of merging all nnn items together each time we merge two sorted sublists.