longluo

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longluo

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longluo

/**
 * Binary Search
 * 二分查找
 * 
 * Returns the index of the specified target in the specified array.
 *
 * @param arr    the array of integers, must be sorted in ascending order
 * @param target the search target
 * @return index of key in array {@code arr} if present; {@code -1} otherwise
 */

longluo

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longluo

/**
 * 判断数字是否是一个质数
 * 
 * check is a prime number
 * 
 */
boolean isPrime(int n) {
    for (int i = 2; i * i <= n; i++) {
        if (n % i == 0) {
            return false;