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Problem solution · C++

Find Longest Self-Contained Substring

Find Longest Self-Contained Substring: a C++ solution using sliding window or two pointers. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Sliding window or two pointers
Source
walkccc LeetCode Solutions
Length
49 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Sliding window or two pointers

For Find Longest Self-Contained Substring, the implementation maintains a moving interval and updates only the information that enters or leaves the window.

  1. Choose the invariant that makes a window valid or useful.
  2. Advance the right boundary and add the new element.
  3. Move the left boundary only as needed while maintaining the invariant and updating the answer.

Code notes

  • 49 lines of C++ from the credited upstream file 3104.cpp.
  • The implementation visibly relies on sequence storage.
  • 4 loop blocks detected.

Complexity

Confirm that neither pointer moves backwards; if so, the scan is usually linear apart from the window’s data-structure operations.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeFind Longest Self-Contained Substring · C++C++
Use this to learn the idea, then write your own version.
class Solution { public:  int maxSubstringLength(string s) {    int ans = -1;    vector<int> count(26);     for (const char c : s)      ++count[c - 'a'];     for (int n = 1; n <= 26; ++n)      ans = max(ans, maxSubstringLengthWithNUniqueLetters(s, n, count));     return ans;  }  private:  // Similar to 395. Longest Substring with At Least K Repeating Characters  int maxSubstringLengthWithNUniqueLetters(const string& s, int n,                                           const vector<int>& allCount) {    int res = -1;    // the number of unique letters    int uniqueLetters = 0;    // the number of letters that have all their frequency in the substring    int lettersHavingAllFreq = 0;    vector<int> count(26);     for (int l = 0, r = 0; r < s.length(); ++r) {      if (++count[s[r] - 'a'] == 1)        ++uniqueLetters;      if (count[s[r] - 'a'] == allCount[s[r] - 'a'])        ++lettersHavingAllFreq;      while (uniqueLetters > n) {        if (count[s[l] - 'a'] == allCount[s[l] - 'a'])          --lettersHavingAllFreq;        if (--count[s[l] - 'a'] == 0)          --uniqueLetters;        ++l;      }      // Since both the number of unique letters and the number of letters      // having all their frequency are equal to n, this is a valid window.      // Implcit: uniqueLetters == n      if (lettersHavingAllFreq == n && r - l + 1 < s.length())        res = max(res, r - l + 1);    }     return res;  }};

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