avc278 - 1.06 - Javascript (#61)

Added my solution to the Chapter 1 Problem 6 folder.

Author: avc278

JavaScript/chapter01/p06_string_compression/avc278.js

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+// String Compression: Implement a method to perform basic string compression using the counts of repeated characters.
+// For example, the string aabcccccaaa would become a2b1c5a3. If the "compressed" string would not become smaller than
+// the original string, your method should return the origin string. You can assume the string has only uppercase and
+// lowercase letters (a-z).
+
+const assert = require("assert");
+
+/**
+ * Returns the compressed version of the string if applicable
+ * @param  {string}  str input string to compress
+ * @return {string}      either the compressed string, or the string
+ *
+ * Storing the outputStrArr is at worst size 2N for an input string whose size is N (still O(N) additional space).
+ * Storing and updating the pointers `idx`, `next`, and `freq` take O(1) additional space and O(1) time.
+ * As we iterate through the str in the outer while loop, in one extreme, assuming no repeating letters in a sequence,
+ * idx increments by 1 each iteration, and so it's O(N) runtime. On the other extreme, assuming all repeating letters,
+ * this would be O(N) on the outer loop, and O(N) in the inner while loop. The trick to knowing this is still O(N)
+ * and not O(N^2) is the fact that we don't visit any one index pointer more than once in our two while loops.
+ * Both loops are still just part of the same loop, which results in O(N).
+ *
+ * Runtime: O(N)
+ * Space:   O(N)
+ *
+ */
+const compress = (str) => {
+  let outputStrArr = [];
+  let idx = 0;
+
+  while (idx < str.length) {
+    const currLetter = str[idx];
+    let freq = 1;
+
+    let next = idx + 1;
+    while (next < str.length && currLetter === str[next]) {
+      next += 1;
+      freq += 1;
+    }
+
+    outputStrArr.push(currLetter);
+    outputStrArr.push(freq);
+    idx = next;
+  }
+
+  return outputStrArr.length < str.length ? outputStrArr.join("") : str;
+};
+
+describe(module.filename, () => {
+  it("should compress a string with repeated characters.", () => {
+    assert.ok(compress("aabcccccaaa", "a2b1c5a3"));
+    assert.ok(compress("aaaaaaaaaaa", "a11"));
+  });
+  it("should not compress a string whose str length is less than the compressed version.", () => {
+    assert.equal(compress("tech"), "tech");
+    assert.equal(compress("abcdefg"), "abcdefg");
+    assert.equal(compress(""), "");
+  });
+});