1    public int myAtoi(String str) {
 2        if(null == str || 0 == str.length()) return 0;
 3        str = str.trim();  //剔除无效字符
 4        char first = str.charAt(0);
 5        if(str.length() == 1 && !Character.isDigit(first)) return 0; //无效字符返回0，比如"+"
 6        boolean isNeg = (first == '-'); //判断第一个字符是否为负号
 7        int idx = isNeg ? 1 : 0;
 8        if(first == '+') idx = 1;
 9        while(str.charAt(idx) == '0'){ //把最前面的0去掉
10         idx++;
11        }
12        int result = 0;
13        while(idx < str.length()){
14         char ch = str.charAt(idx);
15         if(!Character.isDigit(ch)) break; //如果遇到非数字字符，则终止循环，返回已经计算的数字
16         int num = ch - '0'; //char字符转化成对应数字
17         if(isNeg && result > Integer.MAX_VALUE/10) return Integer.MIN_VALUE; //检测整数是否溢出的情况
18         if(isNeg && result == Integer.MAX_VALUE/10 && num > 8) return Integer.MIN_VALUE;
19         if(!isNeg && result > Integer.MAX_VALUE/10) return Integer.MAX_VALUE;
20         if(!isNeg && result == Integer.MAX_VALUE/10 && num > 7) return Integer.MAX_VALUE;
21         result = result * 10 + num;
22         idx++;
23        }
24        return isNeg?(-result):result;
25    }

   public static int parseInt(String s, int radix)throws NumberFormatException{/** WARNING: This method may be invoked early during VM initialization* before IntegerCache is initialized. Care must be taken to not use* the valueOf method.*/if (s == null) {throw new NumberFormatException("null");}if (radix < Character.MIN_RADIX) { //radix参数指定转化为几进制的数throw new NumberFormatException("radix " + radix +" less than Character.MIN_RADIX");}if (radix > Character.MAX_RADIX) {throw new NumberFormatException("radix " + radix +" greater than Character.MAX_RADIX");}int result = 0;boolean negative = false;int i = 0, len = s.length();int limit = -Integer.MAX_VALUE;int multmin;int digit;if (len > 0) {char firstChar = s.charAt(0);if (firstChar < '0') { // Possible leading "+" or "-"if (firstChar == '-') {negative = true; //为负数limit = Integer.MIN_VALUE;} else if (firstChar != '+') //如果第一个字符不是数字也不是‘+’、‘-’符号，则抛出格式异常throw NumberFormatException.forInputString(s);if (len == 1) // Cannot have lone "+" or "-"throw NumberFormatException.forInputString(s);i++;}multmin = limit / radix;while (i < len) {// Accumulating negatively avoids surprises near MAX_VALUEdigit = Character.digit(s.charAt(i++),radix); //转化成对应数字if (digit < 0) {throw NumberFormatException.forInputString(s);}if (result < multmin) {throw NumberFormatException.forInputString(s);}result *= radix;if (result < limit + digit) {throw NumberFormatException.forInputString(s);}result -= digit;} //计算出来是负数} else {throw NumberFormatException.forInputString(s);}return negative ? result : -result; }