Merge pull request #184 from Sugyanee/monotonic-stacks-queues-cpp-problem

Added Trapping Rain Water Solution (Two-Pointer Approach)

Author: admirerr

CPP/Stacks_Queues/trapping_rainwater.cpp

@@ -0,0 +1,68 @@
+/****************************************
+* Problem: Trapping Rain Water
+*
+* Description:
+* Calculates the total amount of rainwater that can be trapped 
+* between bars of varying heights after raining. The algorithm 
+* uses a two-pointer approach that efficiently computes trapped 
+* water by maintaining the maximum heights seen from both ends 
+* and moving inward.
+*
+* Approach:
+* - Initialize two pointers (left and right) at both ends.
+* - Track the maximum height on the left and right sides.
+* - Move the pointer with the smaller height inward.
+* - At each step, calculate trapped water based on the difference 
+*   between the current height and the maximum height on that side.
+*
+* Time Complexity: O(n) - single traversal using two pointers
+* Space Complexity: O(1) - constant extra space
+
+* Sample Output:
+* Trapped Rainwater: 6
+*
+* Explanation:
+* Bars at positions 2, 4, 5, 6, 9, and 10 trap water.
+* The total trapped water is 6 units.
+****************************************/
+
+#include <bits/stdc++.h>
+using namespace std;
+
+// Function to calculate total trapped rainwater
+int calculateTrappedWater(vector<int>& heights) {
+    int n = heights.size();
+    int leftIndex = 0, rightIndex = n - 1; 
+    int totalWater = 0; // Variable to store total trapped water
+    int leftMax = 0, rightMax = 0; // Variables to store maximum height encountered from left and right sides
+
+    while (leftIndex <= rightIndex) {
+        
+        // Compare the current heights to decide which pointer to move
+        if (heights[leftIndex] <= heights[rightIndex]) {
+            if (heights[leftIndex] >= leftMax)
+                leftMax = heights[leftIndex];
+            else
+                totalWater += leftMax - heights[leftIndex];
+
+            leftIndex++;
+        } else {
+            if (heights[rightIndex] >= rightMax)
+                rightMax = heights[rightIndex];
+            else
+                totalWater += rightMax - heights[rightIndex];
+
+            rightIndex--;
+        }
+    }
+
+    return totalWater;
+}
+
+int main() {
+    vector<int> elevationMap = {0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1};
+
+    cout << "Trapped Rainwater: " << calculateTrappedWater(elevationMap) << endl;
+
+    return 0;
+}