Implement atoi to convert a string to an integer.
class Solution {
public int myAtoi(string str) {
int n = str.length();
if(n==0) return 0;
int sign=1,i=0;
while(str[i]==' ') i++;//空格
//符号
if(str[i]=='+') {i++;}
else if(str[i]=='-') {sign=-1;i++;}
//防止溢出(was int, which cause stack flow)
long long ans=0；
for(int j=i ;j<n;j++){
if(str[j]>='0'&&str[j]<='9'){
ans=ans*10+str[j]-'0';
if(ans>INT_MAX)//若溢出
return sign<0? INT_MIN : INT_MAX;
}
else
break;
}
ans *= sign;
return (int)ans;//转换
}
};