Algorithm/MaximumAreaHistogram.cpp at master · vksahu399/Algorithm · GitHub

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#include <bits/stdc++.h>

using namespace std;

vector<int> nsel(vector<int> arr, int n) {

	vector<int> nselIndex;	stack<pair<int, int>> s;

	for(int i = 0; i < n; i++) {

	    while(!s.empty() && s.top().first >= arr[i]) {

			s.pop();

		}

		if(s.empty()) {

		    nselIndex.push_back(-1);

		}

		else  {

		    nselIndex.push_back(s.top().second);

		}

		s.push({arr[i], i});

	}

	return nselIndex;

}

vector<int> nser(vector<int> arr, int n) {

	vector<int> nserIndex;

	stack<pair<int, int>> s;

	for(int i = n - 1; i >= 0; i--) {

	    while(!s.empty() && s.top().first >= arr[i]) {

			s.pop();

		}

		if(s.empty()) {

		    nserIndex.push_back(n);

		}

		else  {

		    nserIndex.push_back(s.top().second);

		}

		s.push({arr[i], i});

	}

	reverse(nserIndex.begin(), nserIndex.end());

	return nserIndex;

}

int maxAreaHistogram(vector<int> arr, int n) {

	vector<int> nselIndex = nsel(arr, n);

	vector<int> nserIndex = nser(arr, n);

	vector<int> ans;

	int max = INT_MIN;

	for(int i = 0; i < n; i++) {

		ans.push_back((nserIndex[i] - nselIndex[i] - 1) * arr[i]);

		if(max < ans[i]) {

			max = ans[i];

		}

	}

	return max;

}

int main() {

	vector<int> arr = {6, 2, 5, 4, 5, 1, 6};

	int ans = maxAreaHistogram(arr, arr.size());

	cout << ans << endl;


}