cs: add «2471. Minimum Number of Operations to Sort a Binary Tree by Level»

URL:	https://leetcode.com/problems/minimum-number-of-operations-to-sort-a-binary-tree-by-level/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/minimum-number-of-operations-to-sort-a-binary-tree-by-level.cs

@@ -0,0 +1,51 @@
+public class Solution {
+    private int GetSwaps(int[] level) {
+        int[] sorted = (int[])level.Clone();
+        Array.Sort(sorted);
+
+        var indices = new Dictionary<int, int>();
+        for (int i = 0; i < level.Length; ++i) {
+            indices[level[i]] = i;
+        }
+
+        var swaps = 0;
+        for (int i = 0; i < level.Length; ++i) {
+            if (level[i] == sorted[i]) {
+                continue;
+            }
+
+            ++swaps;
+            var current = indices[sorted[i]];
+            indices[level[i]] = current;
+            level[current] = level[i];
+        }
+
+        return swaps;
+    }
+
+    public int MinimumOperations(TreeNode root) {
+        var q = new Queue<TreeNode>();
+        q.Enqueue(root);
+
+        var swaps = 0;
+        while (q.Count > 0) {
+            var size = q.Count;
+
+            var level = new int[size];
+            for (int i = 0; i < size; ++i) {
+                var node = q.Dequeue();
+                level[i] = node.val;
+
+                foreach (var n in new TreeNode[] { node.left, node.right }) {
+                    if (n != null) {
+                        q.Enqueue(n);
+                    }
+                }
+            }
+
+            swaps += GetSwaps(level);
+        }
+
+        return swaps;
+    }
+}