update: find next greater for each element in an array

Author: ashokdey

src/_Problems_/next-greater-element/index.js

@@ -0,0 +1,46 @@
+/**
+  You must implement the nextGreaterElement() function.
+  For each element in an array, it finds the next greater element in that array.
+  To keep it simple, the next greater element for the last or maximum value in the array is -1.
+
+  Input: [4, 6, 3, 2, 8, 1]
+  Output: {6, 8, 8, 8, -1, -1}
+ */
+
+const Stack = require('../../_DataStructures_/Stack');
+
+function nextGreaterElement(arr) {
+  if (!Array.isArray(arr)) {
+    throw new Error('Invalid Argument');
+  }
+
+  const s1 = new Stack();
+  const nextGreater = [];
+
+  for (let i = arr.length - 1; i >= 0; i -= 1) {
+    if (s1.peek()) {
+      let top = s1.peek();
+      while (top <= arr[i]) {
+        // if the stack is empty, break the while loop
+        if (!s1.peek()) break;
+        // pop the elements
+        s1.pop();
+        // get the new top
+        top = s1.peek();
+      }
+    }
+
+    if (s1.peek()) {
+      nextGreater[i] = s1.peek();
+    } else {
+      nextGreater[i] = -1;
+    }
+
+    // push the element into the stack
+    s1.push(arr[i]);
+  }
+  return nextGreater;
+}
+
+// eslint-disable-next-line no-console
+console.log(nextGreaterElement([4, 6, 3, 2, 8, 1]));