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1671. Minimum Number of Removals to Make Mountain Array 👍

  • Time: $O(n\log n)$
  • Space: $O(n)$
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class Solution {
 public:
  int minimumMountainRemovals(vector<int>& nums) {
    vector<int> l = lengthOfLIS(nums);
    vector<int> r = reversed(lengthOfLIS(reversed(nums)));
    int maxMountainSeq = 0;

    for (int i = 0; i < nums.size(); ++i)
      if (l[i] > 1 && r[i] > 1)
        maxMountainSeq = max(maxMountainSeq, l[i] + r[i] - 1);

    return nums.size() - maxMountainSeq;
  }

 private:
  vector<int> lengthOfLIS(vector<int> nums) {
    // tail[i] := the min tail of all increasing subseqs having length i + 1
    // It's easy to see that tail must be an increasing array.
    vector<int> tail;
    // dp[i] := length of LIS ending at nums[i]
    vector<int> dp;

    for (const int num : nums) {
      if (tail.empty() || num > tail.back())
        tail.push_back(num);
      else
        tail[firstGreaterEqual(tail, num)] = num;
      dp.push_back(tail.size());
    }

    return dp;
  }

  int firstGreaterEqual(const vector<int>& A, int target) {
    return lower_bound(A.begin(), A.end(), target) - A.begin();
  }

  vector<int> reversed(const vector<int>& nums) {
    return {nums.rbegin(), nums.rend()};
  }
};

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class Solution {
  public int minimumMountainRemovals(int[] nums) {
    int[] l = lengthOfLIS(nums);
    int[] r = reversed(lengthOfLIS(reversed(nums)));
    int maxMountainSeq = 0;

    for (int i = 0; i < nums.length; ++i)
      if (l[i] > 1 && r[i] > 1)
        maxMountainSeq = Math.max(maxMountainSeq, l[i] + r[i] - 1);

    return nums.length - maxMountainSeq;
  }

  private int[] lengthOfLIS(int[] nums) {
    // tail[i] := the min tail of all increasing subseqs with length i + 1
    // It's easy to see that tail must be an increasing array.
    List<Integer> tail = new ArrayList<>();
    // dp[i] := length of LIS ending at nums[i]
    int[] dp = new int[nums.length];

    for (int i = 0; i < nums.length; ++i) {
      final int num = nums[i];
      if (tail.isEmpty() || num > tail.get(tail.size() - 1))
        tail.add(num);
      else
        tail.set(firstGreaterEqual(tail, num), num);
      dp[i] = tail.size();
    }

    return dp;
  }

  private int firstGreaterEqual(List<Integer> A, int target) {
    int l = 0;
    int r = A.size();
    while (l < r) {
      final int m = (l + r) / 2;
      if (A.get(m) >= target)
        r = m;
      else
        l = m + 1;
    }
    return l;
  }

  private int[] reversed(int[] nums) {
    int[] A = nums.clone();
    int l = 0;
    int r = nums.length - 1;

    while (l < r)
      swap(A, l++, r--);

    return A;
  }

  private void swap(int[] A, int i, int j) {
    final int temp = A[i];
    A[i] = A[j];
    A[j] = temp;
  }
}

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class Solution:
  def minimumMountainRemovals(self, nums: List[int]) -> int:
    left = self._lengthOfLIS(nums)
    right = self._lengthOfLIS(nums[::-1])[::-1]
    maxMountainSeq = 0

    for l, r in zip(left, right):
      if l > 1 and r > 1:
        maxMountainSeq = max(maxMountainSeq, l + r - 1)

    return len(nums) - maxMountainSeq

  def _lengthOfLIS(self, nums: List[int]) -> List[int]:
    # tail[i] := the min tail of all increasing subseqs having length i + 1
    # It's easy to see that tail must be an increasing array.
    tail = []
    # dp[i] := length of LIS ending at nums[i]
    dp = []

    for num in nums:
      if not tail or num > tail[-1]:
        tail.append(num)
      else:
        tail[bisect.bisect_left(tail, num)] = num
      dp.append(len(tail))

    return dp