二分查找

二分查找很基础也很常用，但是写起来细节上容易出问题，留存

int binary_search(int a[], int aLen, int num) {
    int left, right, middle;
    left = 0;
    right = aLen - 1;
    
    while (left <= right) {
        middle = (left + right) / 2;
        if (a[middle] == num) {
            return middle;
        }
        else {
            if (a[middle] > num) {
                right = middle - 1;
            }
            else {
                left = middle + 1;
            }
        }
    }
    return -1;
}