Trapping Rainwater

trapping_rainwater.cpp

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+//Trapping rainwater problem 
+// Efficient solution 
+// Expected Time Complexity: O(N).
+// Expected Auxiliary Space: O(N).
+// Asked in Amazon Microsoft Google Flipkart
+#include <bits/stdc++.h>
+
+using namespace std;
+
+int main()
+{
+    int t, n;
+    //test cases
+    cin >> t;
+    while (t--)
+    {
+        //size of array
+        cin >> n;
+        long long arr[n];
+        //adding elements
+        for (int i = 0; i < n; i++)
+        {
+            cin >> arr[i];
+        }
+
+        long long result = 0;
+
+        // maximum element on left and right
+        int left_max = 0, right_max = 0;
+
+        // indices to traverse the array
+        int lo = 0, hi = n - 1;
+
+        while (lo <= hi)
+        {
+            if (arr[lo] < arr[hi])
+            {
+                if (arr[lo] > left_max)
+                    // update max in left
+                    left_max = arr[lo];
+                else
+                    // water on curr element = max - curr
+                    result += left_max - arr[lo];
+                lo++;
+            }
+            else
+            {
+                if (arr[hi] > right_max)
+                    // update right maximum
+                    right_max = arr[hi];
+                else
+                    result += right_max - arr[hi];
+                hi--;
+            }
+        }
+
+        cout << result;
+    }
+    return 0;
+}