# 引言
题目链接：https://leetcode.com/problems/string-to-integer-atoi/description/
# 题目大意
实现一个 atoi 函数。输入一串字符串，扫描字符串，跳过前面的空格，直到遇上数字或正负符号才开始做转换，而再遇到非数字或字符串结束时 ('\0') 才结束转换，并将结果返回。
- Example
Input: "42"
Output: 42
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.
Input: "-91283472332"
Output: -2147483648
Explanation: The number "-91283472332" is out of the range of a 32-bit signed integer.
Thefore INT_MIN (−231) is returned.
# 题解
# 一句话题解
按照题意模拟流程。遇上第一个非空字符判断是否为正负号并标记，后续继续按位来计算结果，当遇到非数字字符，返回当前结果，并加上符号标记 (同时处理下 0 值返回和数据溢出即可)。
复杂度 O(n)
# AC 代码
c++ 版本
class Solution | |
{ | |
public: | |
int myAtoi(string str) | |
{ | |
int len = str.length(); | |
if (len < 1) | |
{ | |
return 0; | |
} | |
int retSymbol = 1; | |
int retNum = 0; | |
int index = 0; | |
while (index < len && str[index] == ' ') | |
{ | |
++index; | |
} | |
if (index < len && (str[index] == '-' || str[index] == '+')) | |
{ | |
retSymbol = 1 - 2 * ('-' == str[index++]); | |
} | |
for (; index < len; ++index) | |
{ | |
if (str[index] < '0' || str[index] > '9') | |
{ | |
break; | |
} | |
if ((retNum > INT_MAX / 10) || (retNum == INT_MAX / 10 && (str[index] - '0') > INT_MAX % 10)) | |
{ | |
return 1 == retSymbol ? INT_MAX : INT_MIN; | |
} | |
retNum = retNum * 10 + str[index] - '0'; | |
} | |
return retSymbol * retNum; | |
} | |
}; |
go 版本
const ( | |
INT32_MAX = int(^uint32(0) >> 1) | |
INT32_MIN = ^INT32_MAX | |
) | |
func myAtoi(str string) int { | |
lens := len(str) | |
if lens < 1 { | |
return 0 | |
} | |
retSymbol, retNum, index := 1, 0, 0 | |
for index < lens && str[index] == ' ' { | |
index++ | |
} | |
if index < lens && str[index] == '+' { | |
index++ | |
} else if index < lens && str[index] == '-' { | |
retSymbol = -1 | |
index++ | |
} | |
for ; index < lens; index++ { | |
if str[index] < '0' || str[index] > '9' { | |
break | |
} | |
if retNum > INT32_MAX/10 || (retNum == INT32_MAX/10 && int(str[index]-'0') > INT32_MAX%10) { | |
if 1 == retSymbol { | |
return INT32_MAX | |
} | |
return INT32_MIN | |
} | |
retNum = retNum*10 + int(str[index]-'0') | |
} | |
return retNum * retSymbol | |
} |
