A very interesting numeric problem. It involves a lot tricks.

Linear search (by +\-) is not feasible - too slow. So binary search is a good idea. Also please take special care of integer overflow!

class Solution {public:    unsigned long _divide(unsigned long tgt, unsigned long adiv, unsigned long start)    {        if ((tgt - start) <= adiv) return (tgt - start) / adiv;                unsigned n = 0;        while (((adiv << n) + start) < tgt) n++;        return (1 << (n - 1)) + _divide(tgt, adiv, (adiv << (n - 1)) + start);    }    int divide(int dividend, int divisor) {        if (dividend == 0) return 0;        unsigned long adend = abs(dividend);        unsigned long adiv = abs(divisor);        int iNeg = 0;        if ((dividend < 0 && divisor > 0) ||            (dividend > 0 && divisor < 0))        {            iNeg = -1;        }        else        {            iNeg = 1;        }                if (adiv == 1)    return adend * iNeg;        unsigned long ret = _divide(adend, adiv, 0);                return ret * iNeg;    }};