Merge K Sorted Lists

## Problem You are given an array of `k` sorted integer arrays. Merge all the arrays into one sorted array and return the **length** of the merged array. Each sub-array is already sorted in non-decreasing order. ## Examples **Example 1:** ``` Input: lists = [[1,4,5],[1,3,4],[2,6]] Output: 8 Explanation: Merged list is [1,1,2,3,4,4,5,6] which has length 8. ``` **Example 2:** ``` Input: lists = [] Output: 0 ``` **Example 3:** ``` Input: lists = [[]] Output: 0 ``` ## Approach Hints Use a min-heap of size `k` to always extract the smallest current element across all lists in O(N log k) time, where N is the total number of elements.