lab1

1. 没有解释思路，因为基本都是看题目要求用几个符号凑凑就好了，除了leftBitCount这个写了好久。虽然是上学期的实验，但是我依然记得这个leftBitCount，好不容易测试对了吧结果符号数超了一个，但是看来看去也是最简版本了，为了减一个符号磕了好久。
2. bitXor

   /* 
    * bitXor - x^y using only ~ and & 
    *   Example: bitXor(4, 5) = 1
    *   Legal ops: ~ &
    *   Max ops: 14
    *   Rating: 1
    */
   int bitXor(int x, int y) 
   {
           int a,b;
           a=~x&y;
           b=x&~y;
           a=~(~a&~b);
           return a;
   }

    

3. minusOne

   /*
    * minusOne - return a value of -1
    *   Legal ops: ! ~ & ^ | + << >>
    *   Max ops: 2
    *   Rating: 1
    */
   int minusOne(void) {
       int a=0;
       a=~a;
       return a;
   }

    

4. leastBitPos

   /*
    * leastBitPos - return a mask that marks the position of the
    *               least significant 1 bit. If x == 0, return 0
    *   Example: leastBitPos(96) = 0x20
    *   Legal ops: ! ~ & ^ | + << >>
    *   Max ops: 6
    *   Rating: 2
    */
   int leastBitPos(int x) {
       int y=~x;
       y=y+0x1;
       y=x&y;
       return y;
   }

    

5. allEvenBits

   /*
    * allEvenBits - return 1 if all even-numbered bits in word set to 1
    *   Examples allEvenBits(0xFFFFFFFE) = 0, allEvenBits(0x55555555) = 1
    *   Legal ops: ! ~ & ^ | + << >>
    *   Max ops: 12
    *   Rating: 2
    */
   int allEvenBits(int x) {
       int a=0x55;
       a+=a<<8;
       a+=a<<16;
       a=a+~(a&x)+1;
       return !a;
   }

    

6. byteSwap

   /*
    * byteSwap - swaps the nth byte and the mth byte
    *  Examples: byteSwap(0x12345678, 1, 3) = 0x56341278
    *            byteSwap(0xDEADBEEF, 0, 2) = 0xDEEFBEAD
    *  You may assume that 0 <= n <= 3, 0 <= m <= 3