Merge pull request #147 from jayalloyd/feat/trapping-rain-water-on

feat(arrays): Add Trapping Rain Water O(n) solution

Author: admirerr

Java/arrays/TrappingRainWaterCode.java

@@ -0,0 +1,42 @@
+public class TrappingRainWaterCode {
+    public static int trappedRainWater(int height[]) {
+        // auxilliary array
+        // calculate left max boundary array
+        int[] leftMaxBound = new int[height.length];
+        leftMaxBound[0] = height[0];
+        for (int i = 1; i < height.length; i++) {
+            leftMaxBound[i] = Math.max(height[i], leftMaxBound[i - 1]);// i starts with 1 since we have already
+                                                                       // calculated 0.compares current height with prev
+                                                                       // leftmax boundary
+
+        }
+
+        // calculate right max boundary
+        int[] rightMaxBound = new int[height.length];
+        rightMaxBound[height.length - 1] = height[height.length - 1];// for last element rightmax boundary will be
+                                                                     // itself
+        for (int i = height.length - 2; i >= 0; i--) {
+            rightMaxBound[i] = Math.max(height[i], rightMaxBound[i + 1]);
+
+        }
+        int trappedWater = 0;
+        // loop
+
+        for (int i = 1; i < height.length; i++) {
+            // water level->mix(left max boundary,right max boundary)
+            int waterLevel = Math.min(leftMaxBound[i], rightMaxBound[i]);
+
+            // trapped water->waterlevel-height[i]
+            trappedWater += waterLevel - height[i];
+        }
+        return trappedWater;
+
+    }
+
+    public static void main(String[] args) {
+        int[] height = { 4, 2, 0, 6, 3, 2, 5 };
+        System.out.println(trappedRainWater(height));
+
+    }
+}
+/* time complexity->is proportional to number of bars */