a. i=j,单字符必定为回文串
b. j-i<=2 ,dp[i][j]=true
c. i和j相差大于一 如果s[i]=s[j]则dp[i][j]=dp[i+1]dp[j-1]
func longestPalindrome(s string) string {
length:=len(s)
if length <=1 {
return s
}
dp := make([][]bool,length)
for i:=0;i<length;i++{
dp[i]=make([]bool,length)
}
star :=0
Maxlen:=1
for i:=0;i<length;i++{
dp[i][i]=true
}
for j:=1;j<length;j++{
for i:=0;i<j;i++{
if s[i]==s[j] {
if j-i<=2{
dp[i][j]=true
}else {
dp[i][j]= dp[i+1][j-1]
}
}
if dp[i][j] && j-i+1 >Maxlen{
Maxlen=j-i+1
star=i
}
}
}
return s[star:star+Maxlen]
}