LeetCode/cs/smallest-range-covering-elements-from-k-lists.cs

public class Solution {
    private struct Number : IComparable<Number> {
        public int Value;
        public int Index;

        public Number(int value, int index) {
            Value = value;
            Index = index;
        }

        public int CompareTo(Number other) => Value.CompareTo(other.Value);
    }

    public int[] SmallestRange(IList<IList<int>> nums) {
        var merged = new List<Number>();
        for (int i = 0; i < nums.Count; ++i) {
            foreach (var num in nums[i]) {
                merged.Add(new Number(num, i));
            }
        }

        merged.Sort();

        var freqs = new Dictionary<int, int>();
        int left = 0, count = 0;
        int start = 0, end = int.MaxValue;

        for (int right = 0; right < merged.Count; ++right) {
            var num = merged[right];

            freqs[num.Index] = 1 + freqs.GetValueOrDefault(num.Index, 0);
            if (freqs[num.Index] == 1) {
                ++count;
            }

            for (; count == nums.Count; ++left) {
                var range = merged[right].Value - merged[left].Value;
                if (range < end - start) {
                    start = merged[left].Value;
                    end = merged[right].Value;
                }

                --freqs[merged[left].Index];
                if (freqs[merged[left].Index] == 0) {
                    --count;
                }
            }
        }

        return [start, end];
    }
}