位运算实现 swap
异或的性质
\[a \oplus a=0
\]
\[a \oplus 0=a
\]
\[a\oplus b=b \oplus a
\]
a = a ^ b;
b = a ^ b;
a = a ^ b;
证明
\[a=(a \oplus b)
\]
\[b=(a \oplus b) \oplus b=a \oplus (b \oplus b) = a \oplus 0 = a
\]
\[a=(a \oplus b) \oplus a =(a \oplus a) \oplus b = 0 \oplus b =b
\]