二分查找法

二分法的前提是：数据量就大，比如在一个很长的字符串中查某个字符；还有就是查找的数据必须是排好序的。

递归

int search(int data, int[] your_array, int low, int high)
{
    int mid = (low+high)/2;
    if (your_array[mid] > data)
    { return search(data, your_array, mid+1, high); }
    else if (your_array[mid] < data)
    { return search(data, your_array, low, mid-1); }
    else if (your_array[mid] == data)
    { return your_array[mid];}
    else return null;
}

迭代
int search(int data, int[] your_array)
{
    int low = 0;
    int high = your_array.Length;
    int mid;
    while(low != high)
    {
        mid = (low + high)/2;
        if (your_array[mid] == data) return data;
        else
        { 
            if (your_array[mid] > data) { high = mid - 1;}
            else if (your_array[mid] < data) { low = mid + 1;}
        }    
    }
    return null;
}