【回文】leetcode - Shortest Palindrome

题目：

Shortest Palindrome

Given a string S, you are allowed to convert it to a palindrome by adding characters in front of it. Find and return the shortest palindrome you can find by performing this transformation.

For example:

Given

"aacecaaa"

, return

"aaacecaaa"

.

Given

"abcd"

, return

"dcbabcd"

.

分析：

利用manacher算法进行求解。时间复杂度O(n)。空间复杂度O(n).

class Solution {
public:
string shortestPalindrome(string s) {
if(s.size()<=1)
return s;

string str(s.size()*2+1,'#');
for(int i=0,j=1;i<s.size();++i,j+=2)
str[j]=s[i];
int res=1,id=1,size=str.size();
vector<int> p(size,1);
p[1]=2;
for(int i=2;i<=size/2;++i)
{
int maxright=p[id]+id-1;
if(i>maxright)
{
//注意检查越界
while(i - p[i]>=0 && str[i+p[i]]==str[i-p[i]])
++p[i];
}
else
{
int idleft=id-p[id]+1;
int k=i-id,j=id-k,tmp=j-p[j]+1;//i和j关于id对称
if(tmp>idleft)
p[i]=p[j];
else if(tmp<idleft)
p[i]=p[id]-k;
else
{
p[i]=p[j];
while(i - p[i]>=0 && str[i+p[i]]==str[i-p[i]])
++p[i];
}

}
if(p[i]+i>p[id]+id)
id=i;
if(i-p[i]+1==0)
res=i;
}

if (res == s.size())
return s;
else
{
string tmp = string(s.begin() + res, s.end());
reverse(tmp.begin(), tmp.end());
return tmp + s;
}
}
};