Update 42_Trapping_Rain_Water.java

Author: seanprashad

Stacks/42_Trapping_Rain_Water.java

@@ -0,0 +1,27 @@
+class Solution {
+    public int trap(int[] height) {
+        if (height == null || height.length == 0) {
+            return 0;
+        }
+
+        int result = 0;
+        Stack<Integer> st = new Stack<>();
+
+        for (int i = 0; i < height.length; i++) {
+            while (!st.isEmpty() && height[i] > height[st.peek()]) {
+                int idx = st.pop();
+
+                if (!st.isEmpty()) {
+                    int leftPillar = st.peek();
+                    int minHeight = Math.min(height[leftPillar], height[i]);
+
+                    result += (minHeight - height[idx]) * (i - leftPillar - 1);
+                }
+            }
+
+            st.push(i);
+        }
+
+        return result;
+    }
+}