输入:s = “babad”
输出:“bab”
/*
暴力破解法n^3
依次判断是否是回文,并记录
*/
class Solution {
public String longestPalindrome(String s) {
char[] charArr = s.toCharArray();
int len = charArr.length;
//比较两个以上的数是否是回文
int begin = 0, maxLen = 1;
for(int i=0; i<len-1; i++)
{
for(int j=i+1; j<len; j++)
{
if( j-i+1 > maxLen && huiWen( charArr,i,j ) )
{
maxLen = j-i+1;
begin = i;
}
}
}
return s.substring( begin, begin+maxLen );
}
//是否是回文
public boolean huiWen( char[] arr, int begin, int end )
{
while( begin < end )
{
if( arr[begin] == arr[end] )
{
begin++;
end--;
}
else
return false;
}
return true;
}
}
/*
中心扩散法 n^2
判断以中心扩散出去的是否是回文
奇数:1个
偶数:2个
*/
class Solution {
public String longestPalindrome(String s) {
char[] charArr = s.toCharArray();
int len = charArr.length;
//确保第二个不越界
String res = s.substring(0,1);
for(int i=0; i<len-1; i++)
{
String oddString = centerFun( charArr, i, i,s );
String ouString = centerFun( charArr, i, i+1,s );
String maxString = oddString.length() > ouString.length() ? oddString : ouString;
if( maxString.length() > res.length() )
res = maxString;
}
return res;
}
public String centerFun( char[] arr, int left, int right, String s )
{
while( left>=0 && right<arr.length )
{
if( arr[left] == arr[right] )
{
left--;
right++;
}
else
break;
}
return s.substring( left+1, right );
}
}