String to Integer (atoi)

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.

public class Solution {
    public int atoi(String str) {
        if (str==null || str.length()==0) {
            return 0;
        }
        //正负号 true是正 false是负
        boolean flag = true;
        int result = 0;
        str = str.trim();
        if (str.charAt(0)=='-') {
            flag = false;
            str = str.substring(1, str.length());
        }
        if (str.charAt(0)=='+') {
            str = str.substring(1, str.length());
        }
        String newstr = new String();
        for (int i=0;i<str.length();i++) {
            if (str.charAt(i)<'0'||str.charAt(i)>'9') {
                break;
            }
            newstr += String.valueOf(str.charAt(i));
        }
        for (int i=0;i<newstr.length();i++) {
            char c = newstr.charAt(newstr.length()-1-i);
            if (result==0 && c=='0') {
                continue;
            }
            if (c<'0'||c>'9') {
                return result;
            }
            int temp = (int)Math.pow(10,i);
            //判断溢出
            if (Integer.MAX_VALUE -  (c-'0')*temp >= result ) {
                result += (c-'0')*temp;
            } else {
                return flag ? Integer.MAX_VALUE : Integer.MIN_VALUE;
            }
        }
        if (flag) {
            return result;
        } else {
            return 0-result;
        }
    }
}