边工作边刷题：70天一遍leetcode: day 42-2

Sum of Two Integers

要点：

• 基本原理就是a^b得到无进位和，a&b得到carry并左移继续计算，这个算法适用于negative因为signed integer都是2’s complement表示的，所以加法都是统一算法。
• 上述算法只适合java，python因为自动整数越界为long。用python要用0x10000000 (33位）做mask取模保持int。

错误点

• 开始最好把a作为res，b作为carry，所以loop条件是b>0
• a和b会变化，所以要把旧值先存下，这点python可以同时赋值

class Solution(object):
    def getSum(self, a, b):
        """
        :type a: int
        :type b: int
        :rtype: int
        """
        MAX_INT = 0x7FFFFFFF
        MIN_INT = 0x80000000
        MASK = 0x100000000
        while b:
            a,b = (a^b) % MASK, ((a&b)<<1 % MASK)
        return a if a<=MAX_INT else (a % MIN_INT)-MIN_INT
        
        # # print bin(a),bin(b)
        # res, carry = a ^ b, (a & b)<<1 # error 2: don't forget shift left
        # while carry:
        #     r = res^carry
        #     #print bin(res), bin(carry)
        #     carry = (res & carry) << 1 # error 1: order: should be last res and last carry
        #     #print bin(res), bin(carry)
        #     res = r
        # return res