Wins the offer-67 string to Integer

Wins the offer-67 string to Integer

Title:
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

Thinking:
divided into two parts, symbols and numerals
but note that:

1. null
2. Empty string ""
3. Only + -
4. There are + -
5. Invaild symbol

Own answer:

public class Solution {
    public int StrToInt(String str) {
        if(str == null || str.length() == 0) return 0;
        char[] ch = str.toCharArray();
        int res = Int(ch);
        return res;
    }
    private int unsignInt(char[] ch, int start){
        int res = 0;
        for(int i = start; i < ch.length; i++){
            if(ch[i] >= '0' && ch[i] <= '9'){
                res *= 10;
                res += (ch[i] - '0');
            }
            else{
                res = 0;
                break;
            }
        }
        return res;
    }
    private int Int(char[] ch){
        boolean isNegtive = false;
        int start = 0;
        if(ch[0] == '+'){
            start++;
        }else if(ch[0] == '-'){
            start++;
            isNegtive = true;
        }
        int res = unsignInt(ch, start);
        if(isNegtive)
            return -res;
        else
            return res;
    }
}

Mistake:
this code does not consider only the sign (although it will directly return 0, but I did not consider itself in an unsigned function to get inside), without considering overflow down

note:

optimization:

public class Solution {
    public int StrToInt(String str) {
        if(str == null || str.length() == 0) return 0;
        char[] ch = str.toCharArray();
        int res = Int(ch);
        return res;
    }
    
    private int Int(char[] ch){
        int isNegtive = 1;
        int start = 0;
        if(ch[0] == '+'){
            start++;
        }else if(ch[0] == '-'){
            start++;
            isNegtive = -1;
        }
        long res = 0;
        for(int i = start; i < ch.length; i++){
            if(ch[i] >= '0' && ch[i] <= '9'){
                res *= 10;
                res += isNegtive * (ch[i] - '0');
                if(res > 0x7fff_ffff || res < 0x8000_0000){
                    res = 0;
                    break;
                }
            }
            else{
                res = 0;
                break;
            }
        }
        
        return (int)res;
    }
}