51. integer into a string

Subject description:

  A string into an integer, functional Integer.valueOf (String), and if the character string does not satisfy the requirement for, such as letters, or other form character string is not a number, 0 is output string appears.

  For example: +123456 Output: 123456

  123a outputs: 0

Analysis of ideas:

  This question is mainly consider the special case character string processing, such as the input string is "", in this form, or the first character of the string is "+" and "-" character.

Code:

public class Test{
    public int stringToInteger(String s){
        if(s==null||s.length()==0)
            return 0;
        if(s.trim().equals(""))  //如果输入的字符串为“   ”
            return 0;
        boolean flag=true;  //用来判断数字的正负
        if(s.charAt(0)=='-')
            flag=false;
        int sum=0;
        for(int i=(s.charAt(0)=='+'||s.charAt(0)=='-')?1:0;i<s.length();i++){  //如果字符串的第一个字符是正负号，那么我们从第二个元素开始遍历，如果不是就从第一个元素遍历。
            if(s.charAt(i)>'0'&&s.charAt(i)<'9'){
                sum=sum*10+(s.charAt(i)-'0');
            }else{
                return 0;
            }
        }
        if(flag==true)
            return sum;
        else
            return -sum;
    }
}