// solution 1, better.
// 思想，先去掉符号，最后再考虑符号。一定要找到一个k, 使 b << k 一定是大于a的，之后再k--
int divide_two_integers(int up, int down) {
long long a = abs((long long)up);
long long b = abs((long long)down);
long long result = 0;
while(a >= b) {
int k = 0;
while(a >= (b << k) ) k++; // gist, should be >= not >
k--;
a -= b << k;
result += 1 << k;
}
return ((up^down)>>31) ? -result : result;
}
// solution 2
int divide_two_integers2(int up, int down) {
long long a = abs((long long)up);
long long b = abs((long long)down);
long long result = 0;
while(a >= b) {
int k = 0;
while(a >= (b << k)) {
result += 1 << k;
a -= b << k;
k++;
}
}
return ((up^down)>>31) ? -result : result;
}