一、001. 整数除法

二分法+快速乘法
https://leetcode.cn/problems/divide-two-integers/solution/shua-chuan-lc-er-fen-bei-zeng-cheng-fa-j-m73b/

    public int divide(int dividend, int divisor) {

        long x=dividend;
        long y=divisor;
        boolean isNeg = false;
        if ((x > 0 && y < 0) || (x < 0 && y > 0)) isNeg = true;
        if (x < 0) x = -x;
        if (y < 0) y = -y;
        long l=0,r=x+1;//左右边界0到x
        while(l<r){//左闭右开
            long mid = (l+r)/2;
            if(mul(mid,y)>x){
                r = mid;
            } else {
                l = mid+1;
            } 
        }
        // long ans = isNeg ? -(l-1L) : (l-1L);
        long ans = isNeg ? -(l-1L) : (l-1L);
        if (ans > Integer.MAX_VALUE || ans < Integer.MIN_VALUE) return Integer.MAX_VALUE;
        return (int)ans;
    }
    long mul(long a,long b){
        long ans = 0;
        while(b>0){//执行b次
            if((b&1)==1){
                ans += a;
            }
            a +=a;
            b>>=1;
        }
        return ans;
    }