Code it. The mergeTwoLists function in my code comes from the problem Merge Two Sorted Lists whose complexity obviously is O(n), n is the sum of length of l1 and l2. To put it simpler, assume the k is 2^x, So the progress of combination is like a full binary tree, from bottom to top. So on every level of tree, the combination complexity is n, because every level have all n numbers without repetition. The level of tree is x, ie log k. So the complexity is O(n log k). for example, 8 ListNode, and the length of every ListNode is x1, x2, x3, x4, x5, x6, x7, x8, total is n. on level 3: x1+x2, x3+x4, x5+x6, x7+x8 sum: n on level 2: x1+x2+x3+x4, x5+x6+x7+x8 sum: n on level 1: x1+x2+x3+x4+x5+x6+x7+x8 sum: n.

Code it. The mergeTwoLists function in my code comes from the problem Merge Two Sorted Lists whose complexity obviously is O(n), n is the sum of length of l1 and l2. To put it simpler, assume the k is 2^x, So the progress of combination is like a full binary tree, from bottom to top. So on every level of tree, the combination complexity is n, because every level have all n numbers without repetition. The level of tree is x, ie log k. So the complexity is O(n log k). for example, 8 ListNode, and the length of every ListNode is x1, x2, x3, x4, x5, x6, x7, x8, total is n. on level 3: x1+x2, x3+x4, x5+x6, x7+x8 sum: n on level 2: x1+x2+x3+x4, x5+x6+x7+x8 sum: n on level 1: x1+x2+x3+x4+x5+x6+x7+x8 sum: n.