Stack

Largest rectangular Area in Histogram

MaxRectangularArea.cpp

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+#include <bits/stdc++.h>
+using namespace std;
+
+//Time Complexity O(n), gave segmentation fault in gfg
+long long getMaxArea(long long arr[], int n)
+    {
+
+    //left subarray
+    vector <long long > left; //stores index of NSL(Next greater to left)
+    stack <pair<long long , int>> s; //pair of NSL and index of NSL
+    int pseudoindex=-1;
+    for(int i=0;i<n;i++)
+    {
+        if(s.empty())
+        left.push_back(pseudoindex);
+
+        else if(!s.empty() && s.top().first<arr[i])
+        left.push_back(s.top().second);
+
+        else if(!s.empty() && s.top().first>=arr[i])
+        {
+            while(!s.empty() && s.top().first>=arr[i])
+            s.pop();
+            if(s.empty())
+            left.push_back(pseudoindex);
+            else
+            left.push_back(s.top().second);
+        }
+        s.push({arr[i],i});
+    }
+
+    //right subarray
+    vector <long long > right; //stores index of NSR
+    stack <pair<long long , int>> st; //pair of NSR and index of NSR
+     pseudoindex=n;
+    for( int i=n-1;i>=0;i--)
+    {
+        if(st.empty())
+        right.push_back(pseudoindex);
+
+        else if(!st.empty() && st.top().first<arr[i])
+        right.push_back(st.top().second);
+
+        else if(!st.empty() && st.top().first>=arr[i])
+        {
+            while(!st.empty() && st.top().first>=arr[i])
+            st.pop();
+            if(st.empty())
+            right.push_back(pseudoindex);
+            else
+            right.push_back(st.top().second);
+        }
+        st.push({arr[i],i});
+    }
+    reverse(right.begin(),right.end());
+
+    long long  width[n],area[n],maxArea=INT_MIN;
+    for(int i=0;i<n;i++)
+    {
+      width[i]=right[i]-left[i]-1;
+      area[i]=width[i]*arr[i];
+      maxArea=max(maxArea,area[i]);
+    }
+    return maxArea;
+}
+
+
+int main()
+{
+    long long t;
+
+    cin>>t;
+    while(t--)
+    {
+        int n;
+        cin>>n;
+
+        long long arr[n];
+        for(int i=0;i<n;i++)
+            cin>>arr[i];
+        Solution ob;
+        cout<<ob.getMaxArea(arr, n)<<endl;
+
+    }
+    return 0;
+}
+
+
+//Another approach, passed gfg
+long long getMaxArea(long long arr[], int n)
+    {
+       stack<int> st;
+       int i;
+       long long area=INT_MIN, maxx= INT_MIN;
+       int temp;
+       for( i=0; i<n; i++)
+       {
+           while( !st.empty()  && arr[st.top()] > arr[i]){
+           temp= st.top();
+            st.pop();
+
+             if(st.empty()){
+                area= arr[temp]* i; //i is rightmost smaller element's index
+            }
+            else{
+                area= arr[temp]* (i-st.top()-1); //st.top() is leftmost smaller element's index
+            }
+            maxx= max(area, maxx);
+           }
+           st.push(i);
+         }
+        while(!st.empty()){
+        temp= st.top();
+        st.pop();
+        if(st.empty()){
+            area= arr[temp]* i;
+        }
+        else{
+            area= arr[temp]* (i-st.top()-1);
+        }
+        maxx= max(area, maxx);
+    }
+    return maxx;
+}