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Time : O(n + n) Space : O(1)
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                            class Solution {
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 public:
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  string reverseWords(string s) {
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    int i, j;
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    for (i = 0, j = 0; i < size(s); i++) {
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      if (s[i] == ' ') {
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        reverse(begin(s) + j, begin(s) + i);
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        j = i + 1;
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      }
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    }
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    reverse(begin(s) + j, end(s));
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    return s;
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  }
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};