Merge pull request #667 from Vivek-K-M/patch-3

Added O(N) time algorithm for max rain water

CPP/tappinrainwater.cpp

@@ -1,24 +1,34 @@
 #include<bits/stdc++.h>
 using namespace std;
 
+//Efficient implementation with time complexity - O(N).
 int maxWater(int arr[], int n)
 {
-
+    int left = 0; 
+    int right = n-1;
     int res = 0;
-
-    for (int i = 1; i < n-1; i++) {
-
-        int left = arr[i];
-        for (int j=0; j<i; j++)
-           left = max(left, arr[j]);
-
-        int right = arr[i];
-        for (int j=i+1; j<n; j++)
-           right = max(right, arr[j]);
-
-       res = res + (min(left, right) - arr[i]);  
+    int maxleft = 0, maxright = 0;
+    while(left <= right)
+    {
+        if(arr[left] <= arr[right])
+        {
+            if(arr[left] >= maxleft) 
+                maxleft = arr[left];
+            else 
+                res += maxleft-arr[left];
+
+            left++;
+        }
+        else
+        {
+            if(arr[right] >= maxright) 
+                maxright = arr[right];
+            else
+                res += maxright-arr[right];
+
+            right--;
+        }
     }
-
     return res;
 }
 
@@ -30,4 +40,4 @@ int main()
     cout << maxWater(arr, n);
 
     return 0;
-}
+}