3115. Maximum Prime Difference 

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class Solution {
    public int maximumPrimeDifference(int[] nums) {
        int i = 0, j = nums.length - 1;
        while (!isPrime(nums[i])) {
            i++;
        }
        while (!isPrime(nums[j])) {
            j--;
        }
        return j - i;
    }

    private boolean isPrime(int num) {
        for (int i = 2; i * i <= num; i++) {
            if (num % i == 0) {
                return false;
            }
        }
        return num != 1; // 注意 1 不是质数
    }
}

Reference

3115. Maximum Prime Difference