Merge Step Functions of (timestamp, value) Pairs

Given two step functions over time, each encoded as sorted [timestamp, value] checkpoints, return the checkpoints of their pointwise sum. The follow-up extends the merge to k functions via divide-and-conquer. Examples Example 1: Input: a = [[1,3],[3,1],[5,3],[6,4],[10,1]], b = [[2,3],[6,3],[11,2]] Output: [[1,6],[2,4],[3,4],[5,6],[6,7],[10,3],[11,2]] Explanation: At each timestamp from the union of checkpoints, sum the active value of a and b. After timestamp 10, a is exhausted and contributes 0, so the final entry inherits b's value 2. Example 2: Input: a = [[5,1]], b = [[5,2]] Output: [[5,3]] Explanation: Both functions end at the same timestamp. Emit one merged entry with the summed value. Example 3: Input: a = [], b = [[1,7],[4,2]] Output: [[1,7],[4,2]] Explanation: When one array is empty, the merged function is just the other array. Constraints 0 <= a.length, b.length <= 10^5 a[i].length == b[j].length == 2 0 <= a[i][0], b[j][0] <= 10^9 (timestamps) -10^4 <= a[i][1], b[j][1] <= 10^4 (values) Within each input array, timestamps are strictly increasing.