Update 42 trapping rain water with O(1) space

java/42-Trapping-Rain-Water.java

@@ -20,3 +20,30 @@ public int trap(int[] heights) {
     }
 
 }
+
+//O(1) space
+class Solution {
+    public int trap(int[] heights) {
+
+        if (heights.length == 0) return 0;
+
+        int l = 0, r = heights.length - 1;
+        int leftMax = heights[l], rightMax = heights[r];
+        int res = 0;
+
+        while (l < r) {
+            if (leftMax < rightMax) {
+                l++;
+                leftMax = Math.max(leftMax, heights[l]);
+                res += leftMax - heights[l];
+            } else {
+                r--;
+                rightMax = Math.max(rightMax, heights[r]);
+                res += rightMax - heights[r];
+            }
+        }
+
+        return res;
+
+    }
+}