LeetCode5 - Longest Palindromic Substring(C++)

Description:

Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

Example 1:

Input: “babad”
Output: “bab”
Note: “aba” is also a valid answer.

Example 2:

Input: “cbbd”
Output: “bb”

AC代碼:

class Solution {
    public:
        string longestPalindrome(string s) {
            if (s.size() <= 1) return s;
            int maxIdx = 0;
            int maxLen = 1;
            int i = 0;

            while (i < s.size()) {
                int start = i;
                int end = i;

                // expand the window from the end if it's an even palindrome
                while (end + 1 < s.size() && s[end] == s[end + 1]) { end++; }
                i = end + 1;

                // expand the window from both sides until it's not longer a palindrome
                while (start - 1 >= 0 && end + 1 < s.size() && s[start - 1] == s[end + 1]) {
                    start--, end++;
                }

                int currLen = end - start + 1;
                if (currLen > maxLen) {
                    maxIdx = start;
                    maxLen = currLen;
                }
            }

            return s.substr(maxIdx, maxLen);
        }
};

測試與結果

int main() {
    Solution s;
    string testString1 = "ababa";
    string result1 = s.longestPalindrome(testString1);
    cout << result1 << endl;

    string testString2 = "cbbd";
    string result2 = s.longestPalindrome(testString2);
    cout << result2 << endl;
}

總結

本題將窗口按照兩個方向進行擴展，如果遇到重複字符，就按照end++的方向擴展，直至不重複爲止。如果不存在重複字符，就嘗試從兩個方向擴展窗口，直至不滿足迴文的條件爲止。