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Problem solution · C++

Longest Almost Palindromic Substring

Longest Almost Palindromic Substring: a C++ solution using binary search. Learn the idea, check the complexity, and read the full code, with credit to Kamyu LeetCode Solutions.

Technique
Binary search
Source
Kamyu LeetCode Solutions
Length
27 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

Binary search

For Longest Almost Palindromic Substring, the implementation exploits a monotonic condition to discard half of the remaining search space after every check.

  1. Identify the ordered answer range or sorted search domain.
  2. Write a predicate whose truth changes only once.
  3. Move the appropriate boundary after each midpoint check and return the final feasible position.

Code notes

  • 27 lines of C++ from the credited upstream file longest-almost-palindromic-substring.cpp.
  • The implementation keeps its working state in language-native values and containers.
  • 3 loop blocks detected.

Complexity

Multiply the logarithmic number of midpoint checks by the cost of one predicate evaluation.

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

Source

Code and credit

This code comes from Kamyu LeetCode Solutions by kamyu104 and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeLongest Almost Palindromic Substring · C++C++
Use this to learn the idea, then write your own version.
// Time:  O(n^2)// Space: O(1) // two pointersclass Solution {public:    int almostPalindromic(string s) {        const auto& expand = [&](int left, int right) {            while (0 <= left && right < size(s) && s[left] == s[right]) {                --left;                ++right;            }            return pair(left, right);        };         int result = 0;        for (int i = 0; i < 2 * size(s) - 1; ++i) {            const auto& [left, right] = expand(i / 2, (i + 1) / 2);            for (const auto& [left, right] : {pair(left - 1, right), pair(left, right + 1)}) {                const auto& [l, r] = expand(left, right);                result = max(result, min((r - l + 1) - 2, static_cast<int>(size(s))));            }        }        return result;    }};

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