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LeetCode 1392: Longest Happy Prefix Solution

Master LeetCode problem 1392 (Longest Happy Prefix), a hard challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

Problem Explanation

Explanation

To find the longest happy prefix of a given string s, we can use the KMP (Knuth-Morris-Pratt) algorithm. The idea is to preprocess s and find the longest prefix suffix array for each index. The longest happy prefix will be the substring from index 0 to the last index of the longest prefix suffix array.

  1. We initialize variables i and j to 1 and 0 respectively.
  2. While iterating through the string:
    • If s.charAt(i) equals s.charAt(j), we increment both i and j.
    • If not, we update j to the value in the prefix suffix array at index j-1.
    • If j reaches 0, we increment i.
  3. The result will be the substring from index 0 to the last index of the prefix suffix array.

Time Complexity: O(n) Space Complexity: O(n)

Solution Code

class Solution {
    public String longestPrefix(String s) {
        int n = s.length();
        int[] lps = new int[n];
        int j = 0;
        for (int i = 1; i < n; i++) {
            if (s.charAt(i) == s.charAt(j)) {
                lps[i] = ++j;
            } else {
                if (j != 0) {
                    j = lps[j - 1];
                    i--;
                }
            }
        }
        return s.substring(0, lps[n - 1]);
    }
}

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