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class Solution {
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  public String removeKdigits(String num, int k) {
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    int n = num.length();
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    if (n == k) {
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      return "0";
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    }
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    Deque<Character> dq = new ArrayDeque<>();
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    for (char ch : num.toCharArray()) {
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      while (!dq.isEmpty() && k > 0 && dq.peekLast() > ch) {
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        dq.pollLast();
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        k--;
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      }
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      dq.addLast(ch);
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    }
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    StringBuilder sb = new StringBuilder();
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    while (!dq.isEmpty() && dq.peekFirst() == '0') {
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      dq.pollFirst();
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    }
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    while (!dq.isEmpty()) {
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      sb.append(dq.pollFirst());
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    }
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    if (k >= sb.length()) {
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      return "0";
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    }
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    return sb.length() == 0 ? "0" : sb.toString().substring(0, sb.length() - k);
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  }
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}