java: add «42. Trapping Rain Water»

URL:	https://leetcode.com/problems/trapping-rain-water/

java/trapping-rain-water.java

@@ -0,0 +1,20 @@
+class Solution {
+  public int trap(int[] height) {
+    var caught = 0;
+
+    int i = 0, j = height.length - 1;
+    int maxLeft = height[i], maxRight = height[j];
+    while (i < j) {
+      maxLeft = Math.max(maxLeft, height[i]);
+      maxRight = Math.max(maxRight, height[j]);
+
+      if (maxLeft < maxRight) {
+        caught += maxLeft - height[i++];
+      } else {
+        caught += maxRight - height[j--];
+      }
+    }
+
+    return caught;
+  }
+}