String - 8. String to Integer (atoi)

8.String to Integer (atoi)

Implement atoi which converts a string to an integer.

The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value.

The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.

If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed.

If no valid conversion could be performed, a zero value is returned.

Note:

• Only the space character ' ' is considered as whitespace character.
• Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−231, 231 − 1]. If the numerical value is out of the range of representable values, INT_MAX (231 − 1) or INT_MIN (−231) is returned.

Example 1:

Input: "42" Output: 42

Example 2:

Input: " -42" Output: -42 Explanation: The first non-whitespace character is '-', which is the minus sign. Then take as many numerical digits as possible, which gets 42.

Example 3:

Input: "4193 with words" Output: 4193 Explanation: Conversion stops at digit '3' as the next character is not a numerical digit.

思路：

题目意思是实现字符串到数字的转换，实现很简单，就是处理各种Corner case，然后对每一位数字字母转换，加起来，这里处理越界情况的时候，不适用更大的存储空间的数据类型，而是直接在一个4字节的类型下处理越界情况，具体就是比如一个正数如果加上比7大的数字，那么就会超过2^31-1，就会越界，负数同理。

代码：

go：

func myAtoi(s string) int {
    var res int
    s = strings.TrimSpace(s)
    
    if s == "" {
    return res
    }

    // 1. 符号问题
    sign := true // 默认为正数
    var start = 0
    if s[0] == '-' {
        start = 1
        sign = false
    } else if s[0] == '+' {
        start = 1
    }

    // 2.0 转换
    for start != len(s) {
        if s[start] < '0' || s[start] > '9' {
            break
        }

        // 2.1 越界问题
        if sign && (res > math.MaxInt32/10 || (res == math.MaxInt32/10 && s[start] > '7')) {
            // 正数一定越界
            return math.MaxInt32
        }
        if !sign && (res > math.MaxInt32/10 || (res == math.MaxInt32/10 && s[start] > '8')) {
            // 负数一定越界
            return math.MinInt32
        }

        res = res*10 + int(s[start]-'0')
        start++
    }
    if !sign {
        return -res
    }

    return res
}