文章目录
1. 题目描述：
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: [−2^31, 2^31 − 1]. If the numerical value is out of the range of representable values, INT_MAX (2^31 − 1) or INT_MIN (−2^31) 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.
Example 4:
Input: “words and 987”
Output: 0
Explanation: The first non-whitespace character is ‘w’, which is not a numerical digit or a +/- sign. Therefore no valid conversion could be performed.
Example 5:
Input: “-91283472332”
Output: -2147483648
Explanation: The number “-91283472332” is out of the range of a 32-bit signed integer.
Thefore INT_MIN (−2^31) is returned.
2. 思路分析：
题目的意思很简单，就是将一个字符串转成整型数字。需要注意的是符号问题还有字符串头部空白问题。
思路就是依次解析每个字符串，然后比较与字符'0'的差值，即该字符表示的数值大小。
注意： 需要判断转换后的数是否越界。
3. Java代码：
源代码：见我GiHub主页
代码：
public static int myAtoi(String str) {
str = str.trim();
if (str == null || str.length() < 1) {
return 0;
}
int i = 0;
// 判断正负号
char flag = '+';
if (str.charAt(0) == '-') {
flag = '-';
i++;
} else if (str.charAt(0) == '+') {
i++;
}
// 用double存储防止越界
double result = 0;
while (i < str.length() && str.charAt(i) >= '0' && str.charAt(i) <= '9') {
result = result * 10 + (str.charAt(i) - '0');
i++;
}
if (flag == '-') {
result = -result;
}
// 判断是否越界
if (result > Integer.MAX_VALUE) {
return Integer.MAX_VALUE;
}
if (result < Integer.MIN_VALUE) {
return Integer.MIN_VALUE;
}
return (int) result;
}