Update 0042-trapping-rain-water.js

Solved trapping-rain-water with O(n) space.

Author: aadil42

javascript/0042-trapping-rain-water.js

@@ -1,3 +1,46 @@
+/**
+ * Linear
+ * Time O(n) | Space O(n) 
+ * https://leetcode.com/problems/trapping-rain-water
+ * @param {number[]} height
+ * @return {number}
+ * 
+ */
+var trap = function(height) {
+    
+    const maxLeft = [];
+    const maxRight = [];
+    const minLeftRight = [];
+
+    let current = 0;
+    for(let i = 0; i < height.length; i++) {
+     maxLeft.push(current);
+     current  = Math.max(current, height[i]);
+    }
+    current = 0;
+    for(let i = height.length - 1; i > -1; i--) {
+        maxRight.push(current);
+        current = Math.max(current, height[i]);
+    }
+    // because the elements were added reverse. 
+    maxRight.reverse();
+
+    for(let i = 0; i < height.length; i++) {
+        const minofLeftRight = Math.min(maxLeft[i],maxRight[i]);
+        minLeftRight.push(minofLeftRight);
+    }
+
+    let water = 0;
+    for(let i = 0; i < height.length; i++) {
+        if(minLeftRight[i] - height[i] > 0) {
+            water += minLeftRight[i] - height[i];
+        }
+    }
+
+    return water;
+};
+
+
 /**
  * https://leetcode.com/problems/trapping-rain-water/
  * Time O(N) | Space O(1)