avc278 - 1.04 - Javascript (#59)

Added my solution to the Chapter 1 Problem 4 folder.

Author: avc278

JavaScript/chapter01/p04_palinperm/avc278.js

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+// Palindrome Permutation: Given a string, write a function to check if it is a permutation of a palindrome. A
+// palindrome is a word or phrase that is the same forwards and backwards. A permutation is a rearrangement of letters.
+// The palindrome does not need to be limited to just dictionary words.
+
+const assert = require("assert");
+
+/**
+ * Checks if the input string can be rearranged into a palindrome
+ * @param  {string} str input string to check against
+ * @return {boolean}    whether the input string can be rearranged into a palindrome
+ *
+ * We don't actually have to rearrange the string to solve this problem; we only need to check its contents.
+ * If the string is of odd  length, then each character in the string has to appear an even number of times, except one.
+ * If the string is of even length, then each character in the string has to appear an even number of times.
+ * We can create a letter frequency map which takes O(N) time to loop through the string, and O(N) space to build.
+ * Looping through the object key-value pairs again is an O(N) operation in the worst case where we loop through it all.
+ *
+ * Runtime: O(N)
+ * Space:   O(N)
+ *
+ */
+const palindromePerm = (str) => {
+  const letterFreqs = {};
+  let length = 0;
+  for (const letter of str) {
+    if (letter === " ") continue;
+
+    letterFreqs[letter.toLowerCase()] =
+      (letterFreqs[letter.toLowerCase()] || 0) + 1;
+    length += 1;
+  }
+
+  let slack = length % 2 == 1;
+  for (const letterFreq of Object.values(letterFreqs)) {
+    if (letterFreq % 2 === 0) continue;
+
+    if (!slack) return false;
+
+    slack = !slack;
+  }
+
+  return true;
+};
+
+describe(module.filename, () => {
+  it("should return true when the input string can be rearranged in the form of a palindrome.", () => {
+    assert.ok(palindromePerm("Tact C o  a     "));
+    assert.ok(palindromePerm("RaCeCaR"));
+  });
+  it("should return false when the input string cannot be rearranged in the form of a palindrome.", () => {
+    assert.ok(!palindromePerm("techqueria"));
+  });
+});