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2434. Using a Robot to Print the Lexicographically Smallest String

  • Time: $O(n)$
  • Space: $O(n)$
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class Solution {
 public:
  string robotWithString(string s) {
    string ans;
    vector<int> count(26);
    stack<char> stack;

    for (const char c : s)
      ++count[c - 'a'];

    for (const char c : s) {
      stack.push(c);
      --count[c - 'a'];
      const char minChar = getMinChar(count);
      while (!stack.empty() && stack.top() <= minChar)
        ans += stack.top(), stack.pop();
    }

    while (!stack.empty())
      ans += stack.top(), stack.pop();

    return ans;
  }

 private:
  char getMinChar(const vector<int>& count) {
    for (int i = 0; i < 26; ++i)
      if (count[i])
        return 'a' + i;
    return 'a';
  }
};

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class Solution {
  public String robotWithString(String s) {
    StringBuilder sb = new StringBuilder();
    int[] count = new int[26];
    Deque<Character> stack = new ArrayDeque<>();

    for (final char c : s.toCharArray())
      ++count[c - 'a'];

    for (final char c : s.toCharArray()) {
      stack.push(c);
      --count[c - 'a'];
      final char minChar = getMinChar(count);
      while (!stack.isEmpty() && stack.peek() <= minChar)
        sb.append(stack.pop());
    }

    while (!stack.isEmpty())
      sb.append(stack.pop());

    return sb.toString();
  }

  private char getMinChar(int[] count) {
    for (int i = 0; i < 26; ++i)
      if (count[i] > 0)
        return (char) ('a' + i);
    return 'a';
  }
}

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class Solution:
  def robotWithString(self, s: str) -> str:
    ans = []
    count = collections.Counter(s)
    stack = []

    for c in s:
      stack.append(c)
      count[c] -= 1
      minChar = self._getMinChar(count)
      while stack and stack[-1] <= minChar:
        ans.append(stack.pop())

    return ''.join(ans + stack[::-1])

  def _getMinChar(self, count: List[int]) -> str:
    for c in string.ascii_lowercase:
      if count[c]:
        return c
    return 'a'