Merge Two Sorted Linked Lists

Given two sorted (non-decreasing) singly linked lists l1 and l2, merge them into one sorted list and return the head. Requirements: Reuse existing nodes (do not allocate a new node per input element; a single dummy node is allowed). Time: O(n+m), Extra space: O(1) (excluding the output list). Example: Input: l1 = [1,2,4], l2 = [1,3,4] Output: [1,1,2,3,4,4] Constraints: 0 <= n, m <= 1e5 Node values in [-1e9, 1e9] Example Input l1=[1,2,4] l2=[1,3,4] Output [1,1,2,3,4,4]