leetcode longest palindromic substring

p[i][j] = 1 
从i开始以j结尾的子串是回文串
p[i][j] != 1
从i开始以j结尾的子串不是回文串

1>
p[i][i] = 1

2>
p[i][i+1] = 1 only if s[i] == s[i+1]

3>
p[i][j] = 1 only if s[i] == s[j] && p[i+1][j-1] == 1