1
/**
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 * @param {string} s
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 * @return {string}
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 */
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var removeDuplicateLetters = function (s) {
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  const lastOccurrence = {};
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  for (let i = 0; i < s.length; i++) {
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    lastOccurrence[s[i]] = i;
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  }
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  const stack = [];
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  const visited = new Set();
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  for (let i = 0; i < s.length; i++) {
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    if (visited.has(s[i])) {
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      continue;
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    }
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    while (
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      stack.length > 0 &&
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      s[i] < stack[stack.length - 1] &&
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      i < lastOccurrence[stack[stack.length - 1]]
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    ) {
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      visited.delete(stack.pop());
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    }
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    visited.add(s[i]);
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    stack.push(s[i]);
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  }
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  return stack.join("");
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};