cs: add «632. Smallest Range Covering Elements from K Lists»

URL:	https://leetcode.com/problems/smallest-range-covering-elements-from-k-lists/
Signed-off-by: Matej Focko <me@mfocko.xyz>

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

@@ -0,0 +1,52 @@
+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];
+    }
+}