ch1-q3.js

'use strict';

/**
 * Count the number of spaces in the string to calculate the new length of the
 * string and move characters back where required replacing spaces with %20.
 *
 * N = |url|
 * Time: O(N)
 * Additional space: O(1)
 *
 * @param  {string[]} url URL string as a character arra which will be updated in place
 * @return {string[]}     Updated URL character array
 */
export function encodeSpaces(url) {
  if (!url || url.length === 0) {
    return url;
  }

  let spaceCount = 0;
  for (let ch of url) {
    if (ch === ' ') {
      ++spaceCount;
    }
  }

  // add an extra 2 characters for each space
  let newLength = url.length - 1 + 2 * spaceCount;
  for (let i = url.length - 1, j = newLength; i >= 0 && j > i; --i, --j) {
    if (url[i] === ' ') {
      url[j] = '0';
      url[--j] = '2';
      url[--j] = '%';
    }
    else {
      url[j] = url[i];
    }
  }

  return url;
}