二分查找算法

public static int binarySearch(int[] arr, int num) {
    return binarySearch(arr, 0, arr.length - 1, num);
}

public static int binarySearch(int[] arr, int left, int right, int num) {
    //如果没有找到
    if (left > right || num < arr[0] || num > arr[arr.length - 1]) {
        return -1;
    }
    //获取中间索引
    int mid = (left + right) / 2;
    //往左边递归找
    if (num < arr[mid]) {
        return binarySearch(arr, left, mid - 1, num);
    }
    //往右边递归找
    else if (num > arr[mid]) {
        return binarySearch(arr, mid + 1, right, num);
    }
    //数据位置找到
    else {
        return mid;
    }
}