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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) {
    const vector<int> l = lengthOfLIS(nums);
    const 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:
  // Similar to 300. Longest Increasing Subsequence
  vector<int> lengthOfLIS(vector<int> nums) {
    // tails[i] := the minimum tail of all the increasing subsequences having
    // length i + 1
    vector<int> tails;
    // dp[i] := the length of LIS ending in nums[i]
    vector<int> dp;
    for (const int num : nums) {
      if (tails.empty() || num > tails.back())
        tails.push_back(num);
      else
        tails[firstGreaterEqual(tails, num)] = num;
      dp.push_back(tails.size());
    }
    return dp;
  }

  int firstGreaterEqual(const vector<int>& A, int target) {
    return ranges::lower_bound(A, 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;
  }

  // Similar to 300. Longest Increasing Subsequence
  private int[] lengthOfLIS(int[] nums) {
    // tails[i] := the minimum tail of all the increasing subsequences with
    // length i + 1
    List<Integer> tails = new ArrayList<>();
    // dp[i] := the length of LIS ending in nums[i]
    int[] dp = new int[nums.length];
    for (int i = 0; i < nums.length; ++i) {
      final int num = nums[i];
      if (tails.isEmpty() || num > tails.get(tails.size() - 1))
        tails.add(num);
      else
        tails.set(firstGreaterEqual(tails, num), num);
      dp[i] = tails.size();
    }
    return dp;
  }

  private int firstGreaterEqual(List<Integer> A, int target) {
    final int i = Collections.binarySearch(A, target);
    return i < 0 ? -i - 1 : i;
  }

  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

  # Similar to 300. Longest Increasing Subsequence
  def _lengthOfLIS(self, nums: list[int]) -> list[int]:
    # tails[i] := the minimum tail of all the increasing subsequences having
    # length i + 1
    tails = []
    # dp[i] := the length of LIS ending in nums[i]
    dp = []
    for num in nums:
      if not tails or num > tails[-1]:
        tails.append(num)
      else:
        tails[bisect.bisect_left(tails, num)] = num
      dp.append(len(tails))
    return dp