8、String to Integer (atoi)

题目：

mplement atoi to convert a string to an integer.

Hint: Carefully consider all possible input cases. If you want a challenge, please do not see below and ask yourself what are the possible input cases.

Notes: It is intended for this problem to be specified vaguely (ie, no given input specs). You are responsible to gather all the input requirements up front.

Update (2015-02-10):

The signature of the

C++

function had been updated. If you still see your function signature
accepts a

const char *

argument, please click the reload button to
reset your code definition.

spoilers alert... click to show requirements for atoi.
解题思路：

1. 字串为空，返回0；

2. 字串的前缀空格需要忽略掉；

3. 遇到的第一个字符，如果是‘+’或‘－’号，将符号标识为相应值；

4. 处理数字；

5. 在上述处理过程中，如果转换出的值超出了int型的范围，就返回int的最大值或最小值。

C++版本：

class Solution {
public:
int myAtoi(string str) {
long long res = 0;
int minus = 1;

if(str!="")
{
while(str.size()>0 and str[0]==' ')    //正负号前面有空格  如"     -123"
str.erase(str.begin());

if(str[0]=='-')       //正负号处理
{
minus=-1;
str.erase(str.begin());
}
else if(str[0]=='+')
{
minus=1;
str.erase(str.begin());
}
}

for(char c : str)
{
if(res>INT_MAX)                    //超出int 范围
break;
else if(isdigit(c))                   //如果是数字
res = 10*res + c-'0';
else                               //特殊符号处理 如"-+123" -> 0
return minus*res;
}

if(minus*res>INT_MAX)
return minus*INT_MAX;
if(minus*res<INT_MIN)
return minus*INT_MIN;

return minus*res;
}
};

python版本：

class Solution(object):
def myAtoi(self, str):
"""
:type str: str
:rtype: int
"""
INT_MAX = 2147483647; INT_MIN = -2147483648
index,res = 0,0
sign = True
if(str==""):return 0;
while(index<len(str) and str[index]==" "):
index += 1
if(index<len(str) and (str[index]=="+" or str[index]=="-")):
if(str[index]=="-"):
sign = False
index += 1
while(index<len(str) and str[index]>='0' and str[index]<='9'):
res = 10*res+int(str[index])
if(res>INT_MAX):
if(sign):return INT_MAX
else:return INT_MIN
index += 1
if(sign==False):return -res
return res