Divide two integers

尼玛考虑负数的情况！而且还有溢出的情况。。。。 -（-2147483648）溢出啦 亲。。。

bit operator的妙用， 左移实现乘2功能

public class Solution {
    public int divide(int dividend, int divisor) {
        long a = Math.abs((long)dividend);
        long b = Math.abs((long)divisor);
        boolean pos = false;
        
        if (dividend > 0 && divisor > 0 
            || dividend < 0 && divisor < 0) {
            pos = true;
        } 
        
        int ans = 0;
        while (a >= b) {
            int shift = 0;
            while ( (b << shift) <= a) {
                shift++;
            }
            ans += (1 << (shift - 1));
            a -= (b << (shift - 1));
        }
        return pos? ans : -ans;
    }
}

如果允许乘法，试着用binary search做了下

//注意dividend是0的情况，这个时候sum总是比被除数大的！
//尼玛考虑负数的情况！而且还有溢出的情况。。。。 -（-2147483648）溢出啦 亲。。。
//要从O(n) -> O(lgn) -> O(1)
public class Solution {
    public int divide(int dividend, int divisor) {
        long a = Math.abs((long)dividend);
        long b = Math.abs((long)divisor);
        boolean pos = false;
        
        if (dividend > 0 && divisor > 0 
            || dividend < 0 && divisor < 0) {
            pos = true;
        } 
        
        int ans = 0;
        long beg = 1;
        long end = a;
        while (beg <= end) {
            long mid = (beg + end)/2;
            if (b * mid <= a && b * (mid + 1) > a) {
                ans = (int)mid;
                break;
            }
            if (b * (mid + 1) <= a) {
                beg = mid + 1;
            } else {
                end = mid - 1;
            }
        }
        return pos? ans : -ans;
    }
}