4

Author: kedebug

TrappingRainWater/TrappingRainWater.cpp

@@ -1,3 +1,4 @@
+// O(n) time, O(n) space
 class Solution {
 public:
     int trap(int A[], int n) {
@@ -23,4 +24,35 @@ class Solution {
         }
         return water;
     }
-};
+};
+
+// O(n) time, O(1) space
+class Solution {
+public:
+    int trap(int A[], int n) {
+        int maxh = 0;
+        int water = 0;
+        int temp = 0;
+        for (int i = 0; i < n; i++) {
+            if (A[i] <= maxh) {
+                temp += maxh - A[i];
+            } else {
+                maxh = A[i];
+                water += temp;
+                temp = 0;
+            }
+        }
+        maxh = 0;
+        temp = 0;
+        for (int i = n - 1; i >= 0; i--) {
+            if (A[i] < maxh) {
+                temp += maxh - A[i];
+            } else {
+                maxh = A[i];
+                water += temp;
+                temp = 0;
+            }
+        }
+        return water;
+    }
+};