Prove safety offer --- 49. Converts the string to integer

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Title Description

Converting a string to an integer, the request can not use the library function converts the integer string. Value of 0 or a character string is not a valid return value 0

Enter a description:

Enter a string including alphanumeric symbols, can be null

Output Description:

If it is a legitimate expression of the digital value is returned, otherwise 0

Example 1 Input

+2147483647
1a33

Export

2147483647
0

Solution one: learning Sao operation of a title in front of a brother

public class Solution {
    public int StrToInt(String str) {
        if(str==null||str.length()==0) return 0;
        try{
            int result = Integer.valueOf(str);
            return result;
        }catch(Exception e){
            return 0;
        }
    }
}

Solution two: write your own traversal conversion

Ideas: feeling a little low

public class Solution {
    public int StrToInt(String str) {
        if(str==null||str.length()==0) return 0;
        char c = str.charAt(0);
        long result;
        boolean flag = true;
        if(c>='0'&&c<='9') result = c-'0';
        else if(c=='+') result = 0;
        else if(c=='-'){
            flag = false;
            result = 0;
        }else return 0;
        if(str.length()>11) return 0;
        for(int i=1;i<str.length();i++){
            c = str.charAt(i);
             if(c>='0'&&c<='9'){
                 result = result*10+(c-'0');
             }else{
                 return 0;
             } 
        }
        if(flag==true&&result<=(long)Integer.MAX_VALUE){
            return (int)result;
        }
        if(flag==false&&(-result)>=(long)Integer.MIN_VALUE){
            return (int)-result;
        }
        return 0;
    }
}

Solution three: elegant code yourself

public class Solution {
    public int StrToInt(String str) {
        if(str==null||str.length()==0) return 0;
        char c = str.charAt(0);
        long result = 0;
        long flag = 1;
        if(c=='-') flag=-1;
        if(str.length()>11) return 0;
        for(int i=((c=='+'||c=='-')?1:0);i<str.length();i++){
            c = str.charAt(i);
             if(c>='0'&&c<='9'){
                 result = (result<<1)+(result<<3)+(c-'0');
             }else{
                 return 0;
             } 
        }
        result = flag*result;
        if(flag==1&&result>(long)Integer.MAX_VALUE) return 0;
        if(flag==-1&&result<(long)Integer.MIN_VALUE) return 0;
        return (int)result;
    }
}