42-trapping-rain-water

42-trapping-rain-water/driver.cpp

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

42-trapping-rain-water/soln.cpp

@@ -0,0 +1,34 @@
+#include "soln.hpp"
+
+// Neetcode's two pointer approach
+int Solution::trap(vector<int> &height)
+{
+	if (height.size() < 3)
+		return 0;
+	int water = 0;
+	int left = 0, right = height.size() - 1;
+	int left_max = height[left], right_max = height[right];
+	bool coming_from_left;
+	while (left < right)
+	{
+		coming_from_left = left_max <= right_max;
+		if (coming_from_left)
+		{
+			left++;
+			water += max(left_max - height[left], 0);
+			left_max = max(left_max, height[left]);
+		}
+		else
+		{
+			right--;
+			water += max(right_max - height[right], 0);
+			right_max = max(right_max, height[right]);
+		}
+	}
+	return water;
+}
+
+bool Solution::test(vector<int> &height, int answer)
+{
+	return trap(height) == answer;
+}

42-trapping-rain-water/soln.hpp

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+#include <bits/stdc++.h>
+using namespace std;
+class Solution
+{
+public:
+	int trap(vector<int> &height);
+	bool test(vector<int> &height, int answer);
+};