二分查找、

public static boolean exist(int[] sortArr, int num) {
    if (sortArr == null || sortArr.length == 0) {
        return false;
    }
    int L = 0;
    int R = sortArr.length - 1;
    int mid = 0;
    while (L < R) {
        mid = L + ((R - L) >> 1);
    }
    return mid == num;
}