三种方法：
1. 对每一个字符，从左右两边开始算
2. 动态规划
string input;
int length = input.length();
int[][] r= new int[length][length];
for (int i = length - 1; i >= 0; i --) {
for (int j = i; j < length; j++) {
if (i == j) {
r[i][j] = 1;
} else if ( input.charAt(i) == input.charAt(j)) {
if (i + 1 == j) {
r[i][j] = 2;
} else if (r[i+1][j - 1] != 0) {
r[i][j] = r[i + 1][ j - 1] + 2;
} else {
r[i][j] = 0;
}
} else {
r[i][j] = 0;
}
}
}
3. Manacher's Algorithmn
https://www.hackerrank.com/topics/manachers-algorithm
http://algs4.cs.princeton.edu/53substring/Manacher.java.html