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Time : O(n) Space : O(1)
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                        class Solution {
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 public:
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  string reverseStr(string s, int k) {
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    int n = size(s);
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    for (int i = 0; i < n; i = i + 2 * k) {
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      int j = i + k - 1, k = i;
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      if (j >= n) j = n - 1;
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      while (k < (j)) {
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        swap(s[k], s[j]);
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        k++, j--;
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      }
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    }
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    return s;
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  }
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};