Create 1718. Construct the Lexicographically Largest Valid Sequence

1718. Construct the Lexicographically Largest Valid Sequence

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+class Solution {
+ public:
+  vector<int> constructDistancedSequence(int n) {
+    vector<int> ans(2 * n - 1);
+    dfs(n, 0, 0, ans);
+    return ans;
+  }
+
+ private:
+  bool dfs(int n, int i, int mask, vector<int>& ans) {
+    if (i == ans.size())
+      return true;
+    if (ans[i] > 0)
+      return dfs(n, i + 1, mask, ans);
+    for (int num = n; num >= 1; --num) {
+      if (mask >> num & 1)
+        continue;
+      if (num == 1) {
+        ans[i] = num;
+        if (dfs(n, i + 1, mask | 1 << num, ans))
+          return true;
+        ans[i] = 0;
+      } else {  
+        if (i + num >= ans.size() || ans[i + num] > 0)
+          continue;
+        ans[i] = num;
+        ans[i + num] = num;
+        if (dfs(n, i + 1, mask | 1 << num, ans))
+          return true;
+        ans[i + num] = 0;
+        ans[i] = 0;
+      }
+    }
+    return false;
+  }
+};