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var findSubstring = function (s, words) {
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  let res = [];
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  let wordLength = words[0].length;
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  let wordCount = words.length;
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  let len = wordCount * wordLength; //Length of sliding window
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  let map = {};
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  for (let word of words) map[word] = map[word] + 1 || 1; //Hash word freq
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  for (let i = 0; i < s.length - len + 1; i++) {
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    let sub = s.slice(i, i + len); //Generate substring of sliding window length
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    if (isConcat(sub, map, wordLength)) res.push(i);
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  }
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  return res;
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};
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function isConcat(sub, map, wordLength) {
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  let seen = {};
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  for (let i = 0; i < sub.length; i += wordLength) {
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    let word = sub.slice(i, i + wordLength);
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    seen[word] = seen[word] + 1 || 1;
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  }
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  for (let key in map) {
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    if (map[key] !== seen[key]) return false; //Word freq must match between map and seen
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  }
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  return true;
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}