Merge k Sorted Lists - D&C Approach

Instead of merging one by one ($O(kN)$ total), divide the lists in half and merge pairwise.

$1.$ Split $k$ lists into two groups of $k/2$.

$2.$ Recursively merge each group.

$3.$ Merge the two resulting lists.

This gives $O(N \log k)$ where $N$ is total elements. Each element is touched $O(\log k)$ times across all merge levels.