折半查找

Binary Search

int binarySearch(int a[], int n, int key)
{
    int low = 0, high = n - 1, mid = 0;
    while(low <= high)
    {
        mid = (low + high) / 2;
        if(a[mid] == key)
            return mid;
        else if(a[mid] < key)
            low = mid + 1;
        else
            high = mid - 1;
    }
    return -1;
}

O(logn)