Added 3 JS solutions for problem 42

javascript/42-Trapping-Rain-Water.js

@@ -0,0 +1,123 @@
+//////////////////////////////////////////////////////////////////////////////
+// Space: O(n)  Time: O(1)  One iteration.
+//////////////////////////////////////////////////////////////////////////////
+
+/**
+ * @param {!Array<number>} heights
+ * @return {number}
+ */
+function trap(heights) {
+
+    let l = 0;
+    let r = heights.length - 1;
+    let lMax = 0;
+    let rMax = 0;
+    let total = 0;
+
+    while (l < r) {
+        if (heights[l] < heights[r]) {
+            if (heights[l] >= lMax) {
+                lMax = heights[l];
+            } else {
+                total += lMax - heights[l];
+            }
+            ++l;
+        } else {
+            if (heights[r] >= rMax) {
+                rMax = heights[r];
+            } else {
+                total += rMax - heights[r];
+            }
+            --r;
+        }
+    }
+
+    return total;
+}
+
+//////////////////////////////////////////////////////////////////////////////
+// Space: O(n)  Time: O(n)  Two iterations (one main loop and one stack).
+//////////////////////////////////////////////////////////////////////////////
+
+/**
+ * @param {!Array<number>} heights
+ * @return {number}
+ */
+function trap(heights) {
+
+    const stack = [];
+    let total = 0;
+
+    for (let i = 0; i < heights.length; ++i) {
+        while (stack.length && heights[i] > heights[top(stack)]) {
+
+            const j = stack.pop();
+
+            if (!stack.length) {
+                break;
+            }
+
+            const k = top(stack);
+            const spread = i - k - 1;
+            const height = Math.min(heights[i], heights[k]) - heights[j];
+            total += spread * height;
+        }
+
+        stack.push(i);
+    }
+
+    return total;
+}
+
+/**
+ * @param {!Array<*>} stack
+ * @return {*}
+ */
+function top(stack) {
+    return stack[stack.length - 1];
+}
+
+//////////////////////////////////////////////////////////////////////////////
+// Space: O(n)  Time: O(n)  Three iterations (two main loop and one stack).
+//////////////////////////////////////////////////////////////////////////////
+
+/**
+ * @param {!Array<number>} heights
+ * @return {number}
+ */
+function trap(heights) {
+
+    let valley = [];
+    let barrier = 0;
+    let trapped = 0;
+
+    for (const height of heights) {
+        if (height >= barrier) {
+            while (valley.length) {
+                trapped += barrier - valley.pop();
+            }
+            barrier = height;
+        } else {
+            valley.push(height);
+        }
+    }
+
+    valley.reverse();
+    valley.push(barrier);
+    heights = valley;
+    valley = [];
+    barrier = 0;
+
+    for (const height of heights) {
+        if (height >= barrier) {
+            while (valley.length) {
+                trapped += barrier - valley.pop();
+            }
+            barrier = height;
+        } else {
+            valley.push(height);
+        }
+    }
+
+    return trapped;
+}