Longest Happy Prefix

Description

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.

Code

Time Complexity: $O(N)$, Space Complexity: $O(N)$

經典 KMP Algorithm 的應用。和 Find the Index of the First Occurrence in a String 同樣解法。

關鍵在於 lps array 的意義。

class Solution {
public:
    string longestPrefix(string s) {
        int n = s.length();
        vector<int> lps(n, 0);
        int len = 0, i = 1;
        while(i < n) {
            if(s[i] == s[len]) {
                lps[i++] = ++len;
            } else {
                if(len == 0) lps[i++] = 0;
                else {
                    len = lps[len - 1];
                }
            }
        }
 
        return s.substr(0, lps.back());
    }
};

Source

• Longest Happy Prefix - LeetCode