1392. Longest Happy Prefix

Problem Statement

A string is called a happy prefix if is a non-empty prefix which is also a suffix (excluding itself).

Given a string s, return the longest happy prefix of s. Return an empty string "" if no such prefix exists.

Example 1:

Input: s = "level"
Output: "l"
Explanation: s contains 4 prefix excluding itself ("l", "le", "lev", "leve"), and suffix ("l", "el", "vel", "evel"). The largest prefix which is also suffix is given by "l".

Example 2:

Input: s = "ababab"
Output: "abab"
Explanation: "abab" is the largest prefix which is also suffix. They can overlap in the original string.

Constraints:

1 <= s.length <= 105
s contains only lowercase English letters.

Intuition

Approach : 
Apply KMP to find largest match of Suffix and Prefix

Video Links

Approach 1:

KMP

C++

class Solution {
public:
    string longestPrefix(string s) {
        vector<int> lps(s.size(), 0);
        int prev=0, i=1;

        while(i<s.size()){
            if(s[i] == s[prev]){
                lps[i] = prev + 1;
                prev++; i++;
            }
            else if(prev == 0){
                lps[i] = 0;
                i++;
            }
            else
                prev = lps[prev-1];
        }

        int last_val = lps.back();

        return s.substr(0, last_val);
    }
};

Approach 2:

C++

Approach 3:

C++

Approach 4:

C++