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/**
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 * @param {string} beginWord
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 * @param {string} endWord
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 * @param {string[]} wordList
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 * @return {number}
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 */
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// Logic is same as One direction, just from both side tword mid, much faster.
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// One Direction solution is here ->
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// https://leetcode.com/problems/word-ladder/discuss/2372376/Easy-Fast-Simple-83.95-235-ms-50.4-MB-One-Direction
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var ladderLength = function (beginWord, endWord, wordList) {
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  const charMap = buildCharMap();
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  const wordSet = new Set(wordList);
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  if (!wordSet.has(endWord)) return 0;
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  let leftSet = new Set([beginWord]),
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    rightSet = new Set([endWord]),
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    level = 1;
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  const helper = (set1, set2) => {
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    const setArr = Array.from(set1);
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    for (let i = 0; i < setArr.length; i++) {
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      const word = setArr[i];
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      for (let i = 0; i < word.length; i++) {
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        for (const c of charMap) {
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          const newWord = word.slice(0, i) + c + word.slice(i + 1);
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          if (set2.has(newWord)) return true;
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          if (wordSet.has(newWord)) {
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            set1.add(newWord);
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            wordSet.delete(newWord);
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          }
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        }
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      }
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      set1.delete(word);
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    }
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  };
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  while (leftSet.size && rightSet.size) {
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    level++;
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    if (helper(leftSet, rightSet)) return level;
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    level++;
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    if (helper(rightSet, leftSet)) return level;
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  }
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  return 0;
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};
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const buildCharMap = () => {
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  const map = [];
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  for (let i = 0; i < 26; i++) {
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    map.push(String.fromCharCode(i + 97));
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  }
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  return map;
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};