[LeetCode]Sum of Two Integers

题目描述：

LeetCode 371. Sum of Two Integers

Calculate the sum of two integers a and b, but you are not allowed to use the operator + and -.

Example:

Given a = 1 and b = 2, return 3.

题目大意：

不使用加减法，计算两个整数a和b的和。

解题思路：

解法I 位运算（Bit Manipulation）异或 + 移位

参考：http://www.geeksforgeeks.org/add-two-numbers-without-using-arithmetic-operators/

Java代码：

public class Solution {
    public int getSum(int a, int b) {
        while (b != 0) {
            int c = a ^ b;
            b = (a & b) << 1;
            a = c;
        }
        return a;
    }
}

解法II 位运算（Bit Manipulation) 模拟加法

Java代码：

public class Solution {
    public int getSum(int a, int b) {
        int r = 0, c = 0, p = 1;
        while ((a | b | c) != 0) {
            if (((a ^ b ^ c) & 1) != 0)
                r |= p;
            p <<= 1;
            c = (a & b | b & c | a & c) & 1;
            a >>>= 1;
            b >>>= 1;
        }
        return r;
    }
}

由于Python没有无符号右移操作，并且当左移操作的结果超过最大整数范围时，会自动将int类型转换为long类型，因此需要对上述代码进行调整。

Python代码（解法I）：

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

Python代码（解法II）：

class Solution0(object):
    def getSum(self, a, b):
        """
        :type a: int
        :type b: int
        :rtype: int
        """
        MAX_INT = 0x7FFFFFFF
        MIN_INT = 0x80000000
        MASK = 0x100000000
        r, c, p = 0, 0, 1
        while a | b | c:
            if (a ^ b ^ c) & 1: r = (r | p) % MASK
            p <<= 1
            c = (a & b | b & c | a & c) & 1
            a = (a >> 1) % MASK
            b = (b >> 1) % MASK
        return r if r <= MAX_INT else (r % MIN_INT) - MIN_INT