[Leetcode]Longest Palindromic Substring

Longest Palindromic Substring

Total Accepted: 82394 Total Submissions: 381112 Difficulty: Medium

Given a string S, find the longest palindromic substring in S. You may assume that the maximum length of S is 1000, and there exists one unique longest palindromic substring.

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使用暴力去做算法的时间复杂度肯定是O(n ^２)，然后有一种叫做manacher的算法。可以把时间复杂度降到O(n)。详细的资料：戳这里

class Solution {
public:
string longestPalindrome(string s) {
int len = s.size();
if(!len)    return "";
string str("$");
for(int i = 0;i <= len - 1;++ i){
str.push_back('#');
str.push_back(s[i]);
}
str.push_back('#');
len = str.size();
int id = 0;
int maxDis = 0;
vector<int> p(len,0);
for(int i = 1;i <= len - 1;++ i){
if(maxDis > i){
p[i] = min(maxDis - i,p[2 * id - i]);
}
for(;str[i - p[i]] == str[i + p[i]];++ p[i])
;
if(i + p[i] > maxDis){
id = i;
maxDis = i + p[i];
}
}
int maxLen = 0;
int pos = 0;
for(int i = 0;i <= p.size() - 1;++ i){
if(p[i] > maxLen){
maxLen = p[i];
pos = i;
}
}
string res(str.substr(pos - maxLen + 1,maxLen * 2 - 1));
string ans;
for(auto ch : res){
if(ch != '#'){
ans.push_back(ch);
}
}
return ans;
}
};