405.md

Question:

Given an integer, write an algorithm to convert it to hexadecimal. For negative integer, two’s complement method is used.

Note:

1. All letters in hexadecimal (a-f) must be in lowercase.
2. The hexadecimal string must not contain extra leading 0s. If the number is zero, it is represented by a single zero character '0'; otherwise, the first character in the hexadecimal string will not be the zero character.
3. The given number is guaranteed to fit within the range of a 32-bit signed integer.
4. You must not use any method provided by the library which converts/formats the number to hex directly.

Example 1:

Input:
26

Output:
"1a"

Example 2:

Input:
-1

Output:
"ffffffff"

Analysis

这道题不难，解法一比较直观，解法二算是一个改进，主要通过hash巧妙地避开了转换函数numToString

Tips

1. num /= 16 可以写成 num >>= 4, 移位更具有效率
2. 固定的16进制转换等，最好用函数数组来快速转换m[num%16]
3. 负数变正数的方法：
   • num += (1 << 32)
   • num += -1&0xffffffff + 1

Code

// 解法一

func numToString(n int) string { if n < 0 || n > 15 { return "" }

    if n < 10 {
            return string(byte('0' + n))
    }

    return string(byte(n - 10 + 'a'))

}

func toHex(num int) string { if num == 0 { return "0" }

    if num < 0 {
            num += -1&0xffffffff + 1
    }

    ret := ""
    for num != 0 {
            ret = numToString(num%16) + ret
            num >>= 4 // num /= 16 位移更有效率
    }
    return ret

}

// 解法二

func toHex2(num int) (answer string) { m := []byte{'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'} if num == 0 { return "0" } if num < 0 { num = (1 << 32) + num } for num > 0 { answer = string(m[num%16]) + answer num >>= 4 } return answer }