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Longest Palindromic Substring.cpp
102 lines (90 loc) · 2.28 KB
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Longest Palindromic Substring.cpp
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// Longest Palindromic Substring
public String longestPalindrome(String s) {
int len = s.length();
int[][] DP = new int[len][len];
// length = 1
for(int i = 0; i < len; i++){
DP[i][i] = 1;
}
// length = 2
for(int i = 0; i < len-1; i++){
if(s.charAt(i) == s.charAt(i+1)){
DP[i][i+1] = 2;
}
}
// length > 2
for(int l = 3; l <= len; l++){
for(int i = 0; i <= len-l; i++){
int j = i + l -1;
if(s.charAt(i) == s.charAt(j) && DP[i+1][j-1] != 0){
DP[i][j] = DP[i+1][j-1]+2;
}
}
}
//find the longest one
int longest = 0;
int start = 0;
int end = 0;
for(int i = 0; i < len; i++){
for(int j = i; j < len; j++){
if(DP[i][j] > longest){
longest = DP[i][j];
start = i;
end = j;
}
}
}
return s.substring(start, end+1);
}
//O(n2) time and O(n2) space dynamic programming
string longestPalindromeDP(string s) {
int n = s.length();
int longestBegin = 0;
int maxLen = 1;
bool table[1000][1000] = {false};
for (int i = 0; i < n; i++) {
table[i][i] = true;
}
for (int i = 0; i < n-1; i++) {
if (s[i] == s[i+1]) {
table[i][i+1] = true;
longestBegin = i;
maxLen = 2;
}
}
for (int len = 3; len <= n; len++) {
for (int i = 0; i < n-len+1; i++) {
int j = i+len-1;
if (s[i] == s[j] && table[i+1][j-1]) {
table[i][j] = true;
longestBegin = i;
maxLen = len;
}
}
}
return s.substr(longestBegin, maxLen);
}
//O(n2) time and O(1) space
string expandAroundCenter(string s, int c1, int c2) {
int l = c1, r = c2;
int n = s.length();
while (l >= 0 && r <= n-1 && s[l] == s[r]) {
l--;
r++;
}
return s.substr(l+1, r-l-1);
}
string longestPalindromeSimple(string s) {
int n = s.length();
if (n == 0) return "";
string longest = s.substr(0, 1); // a single char itself is a palindrome
for (int i = 0; i < n-1; i++) {
string p1 = expandAroundCenter(s, i, i);
if (p1.length() > longest.length())
longest = p1;
string p2 = expandAroundCenter(s, i, i+1);
if (p2.length() > longest.length())
longest = p2;
}
return longest;
}