二分查找

int find(int beginPos,vector<int>& num,int target)
    {
        int endPos = num.size()-1;
        int mid;
        while(beginPos<=endPos)
        {
            mid = beginPos + (endPos - beginPos)/2;
            if(num[mid] == target)
                return mid;
            if(num[mid]<target)
                beginPos = mid +1 ;
            else
                endPos = mid - 1;
        }
        return -1;
    }