leetcode-8-java

Implement atoi which converts a string to an integer.

The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value.

Example 1:

Input: "42"
Output: 42
Example 2:

Input: "   -42"
Output: -42
Explanation: The first non-whitespace character is '-', which is the minus sign.
             Then take as many numerical digits as possible, which gets 42.

/**
    * 1.判断首字符是否是符号, 获取迭代的起始位置
    * 2.判断是否是负号
    * 3.上次迭代的结果数乘以10 然后加上 对应字符数字乘以符号基础数(1 or -1) 
    * 
    * @param str
    * @return
    */
   public static int myAtoi(String str) {
       if (null == str ) {
           return 0;
       }
       str = str.trim();
       if (str.length() == 0) {
           return 0;
       }
       char firstChar = str.charAt(0);
       //是否符号
       boolean isSign = false;
       //是否负号
       boolean isNeg = false;
       if ((isNeg = ('-' == firstChar)) || '+' == firstChar) {
           isSign = true;
       }


       long result = 0;
       //起始位置
       int index = isSign ? 1 : 0;
       //基础数
       int neg = isNeg ? -1 : 1;
       char[] chars = str.toCharArray();
       for (int i = index; i < chars.length; i++) {
           char tmp = chars[i];
           if (tmp < 48 || tmp > 57) {
               break;
           }
           result = result * 10 + Long.valueOf(String.valueOf(tmp)) * neg;
           if (result > Integer.MAX_VALUE) {
               return Integer.MAX_VALUE;
           }
           if (result < Integer.MIN_VALUE) {
               return Integer.MIN_VALUE;
           }
       }
       return (int) result;
   }