[LeetCode] 045: Longest Palindromic Substring

[Problem]
Given a string S, find the longest palindromic substring in S. You may assume that the maximum length of S is 1000, and there exists one unique longest palindromic substring.

[Solution]

class Solution {
public:
	/**
	 * return the longest palindrome between s1[n:1] and s2[1:m]
	 * Example: s1=asdf, s2=fdsb, return sdffds
	 * odd=true
	 * 	s1[n] is the center of the result(the length of the result is odd)
	 * odd=false
	 * 	s1[n]s2[1] is the center of the result(the length of the result is even)
	 */
	string palindrome(string s1, string s2, bool odd){
		// s1 is empty
		if(s1.size() == 0)return "";

		// s1 is not empty, s2 is empty
		if(s2.size() == 0){
			// the length of the result is odd
			if(odd){
				return s1.substr(s1.size()-1, 1);
			}
			// the length of the result is even
			else{
				return "";
			}
		}

		// both s1 and s2 are not empty
		if(odd){
			int i = s1.size()-2, j = 0, len = 0;
			while(i >= 0 && j <= s2.size()-1 && s1[i] == s2[j]){
				i--;
				j++;
				len++;
			}
			return s1.substr(s1.size()-1-len, len+1) + s2.substr(0, len);
		}
		else{
			int i = s1.size()-1, j = 0, len = 0;
			while(i >= 0 && j <= s2.size()-1 && s1[i] == s2[j]){
				i--;
				j++;
				len++;
			}
			return s1.substr(s1.size()-len, len) + s2.substr(0, len);
		}
	}

	/**
	 * get the longest palindrome substr in s
	 */
	string longestPalindrome(string s){
		// Start typing your C/C++ solution below
		// DO NOT write int main() function
		string res = "";
		for(int i = 0; i <= s.size(); ++i){
			string s1 = s.substr(0, i);
			string s2 = s.substr(i, s.size() - i);

			// get longest palindrome in two ways
			string odd = palindrome(s1, s2, true);
			string even = palindrome(s1, s2, false);

			// update result
			res = odd.size() > res.size() ? odd : res;
			res = even.size() > res.size() ? even : res;
		}
		return res;
	}
};

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