题解 | #最长回文子串#

中心扩展法，以（i, j）为中心，从（i，j）以初始长度等于1和初始长度等于2开始向两边扩展

class Solution {
public:

    pair<int, int> expandAroundCenter(const string& A, int left, int right) {
        while(left >= 0 && right < A.size() && A[left] == A[right]) {
            left--;
            right++;
        }
        return {left, right};
    }

    int getLongestPalindrome(string A, int n) {
        // write code here
        int left = 0, right = 0;
        for(int i = 0; i < A.size(); i++) {
            auto [left1, right1] = expandAroundCenter(A, i, i); // 从子串个度为1开始遍历
            auto [left2, right2] = expandAroundCenter(A, i, i + 1); // 从子串长度为2开始遍历
            // 更新回文子串的首尾索引，先更新从长度为1开始遍历的，再更新从长度为2开始遍历的
            if(right1 - left1 > right - left) {
                left = left1;
                right = right1;
            }
            if(right2 - left2 > right - left) {
                left = left2;
                right = right2;
            }
        }

        return A.substr(left + 1, right - 1 - left).size();
    }
};