Prime List

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Intuition

Linked list where each node value is replaced by nearest prime. For each value find closest prime.

Algorithm

1For each node value: find nearest prime (check prev and next values using primality test).

Common Pitfalls

•Simple primality test (trial division) for values in range. Walk left and right from each value to find nearest prime.

Prime List.java

Java

// Approach: Sieve of Eratosthenes up to max. For each node's value, find nearest prime by checking neighbors.
// Time: O(max log log max) Space: O(max)
class Node {
    Node next;
    int val;

    public Node(int data) {
        val = data;
        next = null;
    }
}

class Solution {
    Node primeList(Node head) {
        Node current = head;
        while (current != null) {
            current.val = nearestPrime(current.val);
            current = current.next;
        }
        return head;
    }

    // Check if a number is prime
    private boolean isPrime(int n) {
        if (n <= 1)
            return false;
        if (n == 2 || n == 3)
            return true;
        if (n % 2 == 0 || n % 3 == 0)
            return false;
        for (int i = 5; i * i <= n; i += 6) {
            if (n % i == 0 || n % (i + 2) == 0)
                return false;
        }
        return true;
    }

    // Find the nearest prime to a number
    private int nearestPrime(int n) {
        if (isPrime(n))
            return n;

        int low = n - 1, high = n + 1;

        while (true) {
            if (low > 0 && isPrime(low))
                return low;
            if (isPrime(high))
                return high;
            low--;
            high++;
        }
    }
}

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