原题链接在这里：https://leetcode.com/problems/sum-of-two-integers/
题目：
Calculate the sum of two integers a and b, but you are not allowed to use the operator + and -.
Example 1:
Input: a = 1, b = 2
Output: 3
Example 2:
Input: a = -2, b = 3
Output: 1
题解：
两个数79和16 相加，不考虑进位相加得85, 只考虑进位进位是10, 85+10 = 95正好是结果.
二进制表示时, 不考虑进位0+0=0, 1+0=1, 0+1=1, 1+1=0, 其实就是xor.
只考虑进位0+0=0, 1+0=0, 0+1=0, 1+1=1, 其实就是&.
进位是放到更前以为，所以需要左移动一位, <<1.
可递推调用.
Time Complexity: O(1).
Space: O(1).
AC Java:
1 public class Solution { 2 public int getSum(int a, int b) { 3 if(b == 0){ 4 return a; 5 } 6 int sum = a^b; 7 int carry = (a&b)<<1; 8 return getSum(sum, carry); 9 } 10 }
也可Iteration.
Time Complexity: O(1).
Space: O(1).
AC Java:
1 public class Solution { 2 public int getSum(int a, int b) { 3 while(b != 0){ 4 int carry = (a&b)<<1; 5 a ^= b; 6 b = carry; 7 } 8 return a; 9 } 10 }