12 Maximum Rectangular Area in a Histogram.cpp

// https://practice.geeksforgeeks.org/problems/maximum-rectangular-area-in-a-histogram-1587115620/1#
class Solution
{
  public:
  long long getMaxArea(long long arr[], int n)
    {
    vector<int> left,right;
    stack<pair<int,int>> s1,s2;
    int pseudo_index =-1;
    int pseudo_index1 =n;
    for (int i=0;i<n;i++)
    {
      if (s1.size()==0)
      {
        left.push_back(pseudo_index);
      }
      else if (s1.size()>0 && s1.top().first<arr[i])
      {
        left.push_back(s1.top().second);
      }
      else if (s1.size()>0 && s1.top().first>=arr[i])
      {
         while(s1.size()>0 && s1.top().first>=arr[i])
         {
           s1.pop();
         }
        if (s1.size()==0)
        {
          left.push_back(pseudo_index);
        }
        else
        {
          left.push_back(s1.top().second);
        }
      }
      s1.push({arr[i],i});
    }
    for (int i=n-1;i>=0;i--)
    {
      if (s2.size()==0)
      {
        right.push_back(pseudo_index1);
      }
      else if (s2.size()>0 && s2.top().first<arr[i])
      {
        right.push_back(s2.top().second);
      }
      else if (s2.size()>0 && s2.top().first >= arr[i])
      {
        while(s2.size()>0 && s2.top().first >= arr[i])
        {
          s2.pop();
        }
        if (s2.size()==0)
        {
          right.push_back(pseudo_index1);
        }
        else
        {
          right.push_back(s2.top().second);
        }
      }
      s2.push({arr[i],i});
    }
    reverse(right.begin(),right.end());
    long long m=INT_MIN;
    for (long long i=0;i<n;i++)
    {
      m=max(m,(right[i]-left[i]-1)*arr[i]); // taking max after finding area
    }
    return m;
  }
};