feat: implement optimized two-pointer approach for trapping rainwater (#2975)

dynamic_programming/trapped_rainwater2.cpp

@@ -0,0 +1,111 @@
+/**
+ * @file
+ * @brief Implementation of the [Trapped Rainwater
+ * Problem](https://www.geeksforgeeks.org/trapping-rain-water/)
+ * @details
+ * This implementation calculates the total trapped rainwater using a
+ * two-pointer approach. It maintains two pointers (`left` and `right`) and
+ * tracks the maximum height seen so far from both ends (`leftMax` and
+ * `rightMax`). At each step, the algorithm decides which side to process based
+ * on which boundary is smaller, ensuring O(n) time and O(1) space complexity.
+ * @author [kanavgoyal898](https://github.com/kanavgoyal898)
+ */
+
+#include <algorithm>  /// For std::min and std::max
+#include <cassert>    /// For assert
+#include <cstddef>    /// For std::size_t
+#include <cstdint>    /// For std::uint32_t
+#include <vector>     /// For std::vector
+
+/*
+ * @namespace
+ * @brief Dynamic Programming Algorithms
+ */
+namespace dynamic_programming {
+/**
+ * @brief Function to calculate the trapped rainwater
+ * @param heights Array representing the heights of walls
+ * @return The amount of trapped rainwater
+ */
+uint32_t trappedRainwater(const std::vector<uint32_t>& heights) {
+    std::size_t n = heights.size();
+    if (n <= 2)
+        return 0;  // Not enough walls to trap water
+
+    std::size_t left = 0, right = n - 1;
+    uint32_t leftMax = 0, rightMax = 0, trappedWater = 0;
+
+    // Traverse from both ends towards the center
+    while (left < right) {
+        if (heights[left] < heights[right]) {
+            // Water trapped depends on the tallest wall to the left
+            if (heights[left] >= leftMax)
+                leftMax = heights[left];  // Update left max
+            else
+                trappedWater +=
+                    leftMax - heights[left];  // Water trapped at current left
+            ++left;
+        } else {
+            // Water trapped depends on the tallest wall to the right
+            if (heights[right] >= rightMax)
+                rightMax = heights[right];  // Update right max
+            else
+                trappedWater +=
+                    rightMax -
+                    heights[right];  // Water trapped at current right
+            --right;
+        }
+    }
+
+    return trappedWater;
+}
+
+}  // namespace dynamic_programming
+
+/**
+ * @brief Self-test implementations
+ * @returns void
+ */
+static void test() {
+    std::vector<uint32_t> test_basic = {0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1};
+    assert(dynamic_programming::trappedRainwater(test_basic) == 6);
+
+    std::vector<uint32_t> test_peak_under_water = {3, 0, 2, 0, 4};
+    assert(dynamic_programming::trappedRainwater(test_peak_under_water) == 7);
+
+    std::vector<uint32_t> test_bucket = {5, 1, 5};
+    assert(dynamic_programming::trappedRainwater(test_bucket) == 4);
+
+    std::vector<uint32_t> test_skewed_bucket = {4, 1, 5};
+    assert(dynamic_programming::trappedRainwater(test_skewed_bucket) == 3);
+
+    std::vector<uint32_t> test_empty = {};
+    assert(dynamic_programming::trappedRainwater(test_empty) == 0);
+
+    std::vector<uint32_t> test_flat = {0, 0, 0, 0, 0};
+    assert(dynamic_programming::trappedRainwater(test_flat) == 0);
+
+    std::vector<uint32_t> test_no_trapped_water = {1, 1, 2, 4, 0, 0, 0};
+    assert(dynamic_programming::trappedRainwater(test_no_trapped_water) == 0);
+
+    std::vector<uint32_t> test_single_elevation = {5};
+    assert(dynamic_programming::trappedRainwater(test_single_elevation) == 0);
+
+    std::vector<uint32_t> test_two_point_elevation = {5, 1};
+    assert(dynamic_programming::trappedRainwater(test_two_point_elevation) ==
+           0);
+
+    std::vector<uint32_t> test_large_elevation_map_difference = {5, 1, 6, 1,
+                                                                 7, 1, 8};
+    assert(dynamic_programming::trappedRainwater(
+               test_large_elevation_map_difference) == 15);
+}
+
+/**
+ * @brief Main function
+ * @returns 0 on exit
+ */
+int main() {
+    test();  // run self-test implementations
+    return 0;
+}