Longest Palindromic Substring - Amazon Top Interview Questions
Problem Statement :
Given a string s, return the length of the longest palindromic substring. Constraints n ≤ 1,000 where n is the length of s Example 1 Input s = "mactacocatbook" Output 7 Explanation "tacocat" in the middle is the longest palindromic substring.
Solution :
Solution in C++ :
class Solution {
vector<vector<int>> memo;
int dp(string &s, int i, int j) {
if (i > j) return 0;
if (i == j) return 1;
if (memo[i][j] != -1) // Return precaluclated value.
return memo[i][j];
// When s[i]==s[j] and the maximum palindrome in between is of length j-i-1 (This means
// string i+1,j-1 is a palindrome) return current length.
if (s[i] == s[j] && dp(s, i + 1, j - 1) == j - 1 - i) return memo[i][j] = j - i + 1;
// When current substring is not a pallindrome return max palindrome between (i,j-1) and
// (i+1,j)
return memo[i][j] = max(dp(s, i, j - 1), dp(s, i + 1, j));
}
public:
int solve(string &s) {
memo.resize(s.length(), vector<int>(s.length(), -1));
return dp(s, 0, s.length() - 1);
}
};
int solve(string s) {
return Solution().solve(s);
}
Solution in Java :
import java.util.*;
class Solution {
public int solve(String s) {
int res = 0;
for (int i = 0; i < s.length(); i++) {
int len1 = helper(s, i, i);
int len2 = helper(s, i, i + 1);
res = Math.max(len1, Math.max(len2, res));
}
return res;
}
public int helper(String s, int l, int r) {
while (l >= 0 && r < s.length() && s.charAt(l) == s.charAt(r)) {
l--;
r++;
}
return r - l - 1;
}
}
Solution in Python :
def manacher(s):
t = "#".join([""] + list(s) + [""])
m = len(t)
a = [0] * m
l = 0
r = -1
for i in range(m):
j = 1 if i > r else min(r - i + 1, a[l + r - i])
while i >= j and i + j < m and t[i - j] == t[i + j]:
j += 1
a[i] = j
j -= 1
if i + j > r:
l = i - j
r = i + j
return a
class Solution:
def solve(self, s):
return max(manacher(s)) - 1
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