src/main/java/com/fishercoder/solutions/_632.java

package com.fishercoder.solutions;

import java.util.List;
import java.util.PriorityQueue;

public class _632 {
    public static class Solution1 {
        /**
         * reference: https://discuss.leetcode.com/topic/94445/java-code-using-priorityqueue-similar-to-merge-k-array/2
         */
        public int[] smallestRange(List<List<Integer>> nums) {
            PriorityQueue<int[]> minHeap = new PriorityQueue<>(nums.size(), (a, b) -> a[0] - b[0]);
            /**int[] array consists of three numbers: value; which list in nums; index of value in this list*/

            int max = nums.get(0).get(0);
            for (int i = 0; i < nums.size(); i++) {
                minHeap.offer(new int[]{nums.get(i).get(0), i, 0});
                max = Math.max(max, nums.get(i).get(0));
            }
            int minRange = Integer.MAX_VALUE;
            int start = -1;
            while (minHeap.size() == nums.size()) {
                int[] curr = minHeap.poll();
                if (max - curr[0] < minRange) {
                    minRange = max - curr[0];
                    start = curr[0];
                }
                if (curr[2] + 1 < nums.get(curr[1]).size()) {
                    curr[0] = nums.get(curr[1]).get(curr[2] + 1);
                    curr[2]++;
                    minHeap.offer(curr);
                    max = Math.max(max, curr[0]);
                }
            }
            return new int[]{start, start + minRange};
        }
    }
}