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

Example:

Input: "babad"

Output: "bab"

Note: "aba" is also a valid answer.

Example:

Input: "cbbd"

Output: "bb"

题目和第三题类似，求给定字符串中最大回文字符串。
第一次提交出现TLE（超时），后将第二层循环初始条件j:=i+1改为j:=i+len(output)，既小于已知最长结果的字符串都不再检测，通过。
解法如下：

func longestPalindrome(s string) string {
    var output []byte
    bs := []byte(s)
    if len(bs) > 0 {
        output = bs[0:1]
    }
    for i := 0; i < len(bs); i++ {
        for j := i + len(output); j < len(bs); j++ {
            flag := 0
            for k := 0; k < (j-i+1)/2; k++ {
                if bs[k+i] != bs[j-k] {
                    flag = 1
                    break
                }
            }
            if flag == 0 && j-i+1 > len(output) {
                output = bs[i : j+1]
            }
        }
    }
    return string(output)
}