5. Longest Palindromic Substring

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 code:

class Solution {
public:
    string longestPalindrome(string s) {
        int max_left, left, right, max_len, i;
        max_left = 0;
        max_len = 1;
        for (i = 0; i < s.length()-1 && s.length()-i > max_len/2; ) {
            left = right = i;
            while (right < s.length()-1 && s[right] == s[right+1]) 
                right++;
            i = right+1;
            while (right < s.length()-1 && left > 0 && s[right+1] == s[left-1]) {
                right++;
                left--;
            }
            if (max_len <= right-left+1) {
                max_left = left;
                max_len = right-left+1;
            }
        }
        return s.substr(max_left, max_len);
    }
};

std::string::substr

string substr (size_t pos = 0, size_t len = npos) const;

Generate substring

Returns a newly constructed string object with its value initialized to a copy of a substring of this object.

The substring is the portion of the object that starts at character position pos and spans len characters (or until the end of the string, whichever comes first).

Parameters

pos

Position of the first character to be copied as a substring.
If this is equal to the string length, the function returns an empty string.
If this is greater than the string length, it throws out_of_range.
Note: The first character is denoted by a value of 0 (not 1).

len

Number of characters to include in the substring (if the string is shorter, as many characters as possible are used).
A value of string::npos indicates all characters until the end of the string.

size_t is an unsigned integral type (the same as member type string::size_type).