String to Integer (atoi)

private static final int maxDiv10 = Integer.MAX_VALUE / 10;

public int atoi(String str) {
    int i = 0;
    int n = str.length();
    // 首先忽略字符串前面的空格
    while(i < n && Character.isWhitespace(str.charAt(i))) {
        i++;
    }

    int sign = 1;
    if(i < n && str.charAt(i) == '+') {
        i++;
    } else if(i < n && str.charAt(i) == '-') {
        sign = -1;
        i++;
    }

    int num = 0;
    while(i < n && Character.isDigit(str.charAt(i))) {
        int digit = Character.getNumericValue(str.charAt(i));
        if(num > maxDiv10 || num == maxDiv10 && digit >= 8) {
            return sign == 1?Integer.MAX_VALUE:Integer.MIN_VALUE;
        }

        num = num * 10 + digit;
        i++;
    }

    return sign * num;
}