added working solution for #1243.

problems/array-transformation/README.md

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+# Array Transformation
+
+LeetCode #: [1243](https://leetcode.com/problems/array-transformation/)
+
+Difficulty: Easy
+
+Topic: Array.

problems/array-transformation/solution.js

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+const transformArray = (arr) => {
+  if (arr.length <= 2) {
+    return arr
+  }
+
+  const result = arr.slice()
+  let changed
+  let init
+
+  do {
+    changed = false
+    init = result.slice()
+
+    for (let i = 1; i < init.length - 1; i++) {
+      const element = init[i]
+      const left = init[i - 1]
+      const right = init[i + 1]
+
+      if (element > left && element > right) {
+        result[i] = element - 1
+        changed = true
+      } else if (element < left && element < right) {
+        result[i] = element + 1
+        changed = true
+      }
+    }
+  } while (changed)
+
+  return result
+}
+
+module.exports = transformArray

problems/array-transformation/solution.test.js

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+const transformArray = require('./solution')
+
+test('Example 1', () => {
+  const arr = [6, 2, 3, 4]
+
+  const result = transformArray(arr)
+
+  expect(result).toStrictEqual([6, 3, 3, 4])
+})
+
+test('Example 2', () => {
+  const arr = [1, 6, 3, 4, 3, 5]
+
+  const result = transformArray(arr)
+
+  expect(result).toStrictEqual([1, 4, 4, 4, 4, 5])
+})
+
+test('Array length 1, should return same thing', () => {
+  const arr = [5]
+
+  const result = transformArray(arr)
+
+  expect(result).toStrictEqual([5])
+})
+
+test('Array length 2, should return same thing', () => {
+  const arr = [5, 3]
+
+  const result = transformArray(arr)
+
+  expect(result).toStrictEqual([5, 3])
+})
+
+test('not bigger or smaller on both side, should return same thing', () => {
+  const arr = [1, 2, 2, 3, 3, 4]
+
+  const result = transformArray(arr)
+
+  expect(result).toStrictEqual([1, 2, 2, 3, 3, 4])
+})
+
+test('not bigger or smaller on both side, values higher than first and last element, should return same thing', () => {
+  const arr = [1, 55, 55, 33, 33, 10]
+
+  const result = transformArray(arr)
+
+  expect(result).toStrictEqual([1, 55, 55, 33, 33, 10])
+})