subject
Please write a function StrToInt to convert string to integer. Of course, you can't use atoi or other similar library functions.

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thinking
The topic is very simple, mainly to realize the conversion of each character into a number, and the accumulation can be done. However, there are many special situations that need to be considered, such as null, empty string, with sign, character is not a number, overflow, etc.

For illegal special input, the return value is 0, and a global variable is used to mark it.

When writing code, be sure to consider all kinds of test cases.

package offer;

import org.junit.Test;

/**
 * Convert a string to an integer (implement the function of Integer.valueOf(string), but return 0 if the string does not meet the number requirements).
 * Library functions that require integers that cannot be converted using strings. If the value is 0 or the string is not a legal value, 0 will be returned.
 * @author kankan
 * @creater 2019-07-24 20:16
 */
public class Solution67 {

    public  int StrToInt(String str) {
        //Judge whether the input is legal
        if (null == str || str.trim().equals("")){
            return 0;
        }
        // symbol=0, indicating that the number is positive; symbol=1, indicating that the number is negative; start is used to distinguish whether the first bit is a sign bit.
        int symbol = 0;
        int start = 0;
        char[] chars = str.trim().toCharArray();
        if ('+' == chars[0]){
            start = 1;
        }else if ('-' == chars[0]){
            start = 1;
            symbol = 1;
        }
        int result = 0;
        for (int i = start; i < chars.length; i++) {
            if (chars[i] > '9' || chars[i] < '0'){
                return 0;
            }
            int sum = result * 10 + (int) (chars[i] - '0');
            if ((sum - (int)(chars[i] - '0')) / 10 != result){
                return 0;
            }
            result=result * 10 + (int) (chars[i] - '0');
            /*
             * I think it's wrong to judge whether overflow is the most popular one in java, for example
             * When the input is value=2147483648, the representation inside the computer should be - 2147483648
             * Obviously, value > integer.max'value does not hold.
            */
        }
        // Note: the n-th power of - 1 in java cannot be used: (- 1)^n. '^' exclusive or operation
        // Note that when value=-2147483648, value=-value
        result = (int) Math.pow(-1, symbol) * result;
        return result;
    }

    // 1. Function test (positive, negative, zero, number with sign)
    @Test
    public void testA(){
        System.out.println(new Solution67().StrToInt("1948243")==1948243);

        System.out.println(new Solution67().StrToInt("+1948243")==1948243);

        System.out.println(new Solution67().StrToInt("-1948243")==-1948243);

        System.out.println(new Solution67().StrToInt("-0")==0);

    }
    // 2. Boundary value test (maximum positive integer, minimum negative integer)
    @Test
    public void testB(){
        String str = "+2147483647";
        int strToInt = new Solution67().StrToInt(str);
        System.out.println(strToInt);

        String str1 = "-2147483648";
        int strToInt1 = new Solution67().StrToInt(str1);
        System.out.println(strToInt1);
    }
    // 3. Special test (null, null string, only sign, illegal character)
    @Test
    public void testC(){
        System.out.println(new Solution67().StrToInt(""));

        System.out.println(new Solution67().StrToInt(null));

        System.out.println(new Solution67().StrToInt("1a33"));

        System.out.println(new Solution67().StrToInt("+++"));
    }
}