题目描述：
Implement 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.
就是完成字符串转整数的操作。
本题的复杂点在于情况很多需要考虑：
1.可能是负数。
2.首字母为+
3.跳过开始的'0'
4.超大整数（使用double来解析）public class Solution {
public int MyAtoi(string str) {
if(string.IsNullOrWhiteSpace(str)){
return 0;
}
str = str.Trim();
var digits = "0123456789";
if(str.Length > 1 && !digits.Contains(str[0]) && !digits.Contains(str[1])){
return 0;
}
var s1 = "";
var c = 0;
if(str[0] == '-'){
s1 = "-";
c++;
}
// skip '+'
if(str[0] == '+'){
c++;
}
//skip all starting '0'
while(c < str.Length && str[c] == '0'){
c ++;
}
// start from non-zero number , collect all valid digit char
while(c < str.Length && digits.Contains(str[c])){
s1 += str[c];
c++;
}
double v;
if(double.TryParse(s1, out v)){
if(v > int.MaxValue){
return int.MaxValue;
}
if(v < int.MinValue){
return int.MinValue;
}
return (int)v;
}
return 0;
}
}