Longest Palindromic Substring LeetCode solution in C++

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class Solution {
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 public:
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  string longestPalindrome(string s) {
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    if (s.size() <= 1) return s;
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    string longest = "";
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    for (int i = 0; i < s.size(); i++) {
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      string sub1 = expand(s, i, i + 1);
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      string sub2 = expand(s, i, i);
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      string sub3 = sub1.size() > sub2.size() ? sub1 : sub2;
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      if (sub3.size() > longest.size()) {
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        longest = sub3;
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      }
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    }
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    return longest;
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  }
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  string expand(string s, int i, int j) {
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    while (j < s.size() && i >= 0 && s[i] == s[j]) {
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      i--;
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      j++;
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    }
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    // Add 1 to i and subtract 1 from j because the range is expanded by 1 on
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    // each side before it ends
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    return s.substr(i + 1, j - i - 1);
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  }
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};