day 36: 84-largest-rectangle-in-histogram

84-largest-rectangle-in-histogram/driver.cpp

@@ -0,0 +1,10 @@
+#include "soln.hpp"
+
+int main()
+{
+	vector<int> heights{2, 1, 5, 6, 2, 3};
+	int answer = 10;
+	Solution soln;
+	cout << "Found largest rectangle area correctly? " << soln.test(heights, answer) << endl;
+	return 0;
+}

84-largest-rectangle-in-histogram/soln.cpp

@@ -0,0 +1,57 @@
+#include "soln.hpp"
+
+int Solution::largestRectangleArea(vector<int> &heights)
+{
+	if (heights.size() == 1)
+	{
+		return heights[0];
+	}
+	stack<pair<int, int>> stk;
+	int max_area = 0, max_area_old;
+	for (int i = 0; i < heights.size(); i++)
+	{
+		// empty stk, or increasing => push to stk
+		// cout << i << " " << heights[i] << endl;
+		if (stk.empty())
+		{
+			stk.push({i, heights[i]});
+			// cout << "stack empty" << endl;
+		}
+		else if (heights[i] >= stk.top().second)
+		{
+			stk.push({i, heights[i]});
+			// cout << "greater height, pushing" << endl;
+		}
+		// fun
+		else
+		{
+			int new_top_index = i;
+			// cout << "lower height" << endl;
+			while (!stk.empty() && heights[i] < stk.top().second)
+			{
+				// cout << "popping " << stk.top().first << " " << stk.top().second << endl;
+				max_area_old = (i - stk.top().first) * stk.top().second;
+				max_area = max(max_area, max_area_old);
+				new_top_index = stk.top().first;
+				stk.pop();
+			}
+			stk.push({new_top_index, heights[i]});
+		}
+		// cout << "maxarea = " << max_area << endl;
+	}
+
+	// cout << "non-empty stack at end, popping" << endl;
+	while (!stk.empty())
+	{
+		// cout << stk.top().first << " " << stk.top().second << endl;
+		max_area_old = (heights.size() - stk.top().first) * stk.top().second;
+		max_area = max(max_area, max_area_old);
+		stk.pop();
+	}
+	return max_area;
+}
+
+bool Solution::test(vector<int> &heights, int answer)
+{
+	return largestRectangleArea(heights) == answer;
+}

84-largest-rectangle-in-histogram/soln.hpp

@@ -0,0 +1,8 @@
+#include <bits/stdc++.h>
+using namespace std;
+class Solution
+{
+public:
+    int largestRectangleArea(vector<int>& heights);
+	bool test(vector<int>&heights, int answer);
+};