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.

The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.

If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed.

If no valid conversion could be performed, a zero value is returned. If the correct value is out of the range of representable values, INT_MAX (2147483647) or INT_MIN (-2147483648) is returned.

解法1: 

　　这题只需要考虑数字和符号的情况：

　　1. 若字符串开头是空格，则跳过所有空格，到第一个非空格字符，如果没有，则返回0.

　　2. 若第一个非空格字符是符号+/-，则标记sign的真假，这道题还有个局限性，那就是在c++里面，+-1和-+1都是认可的，都是-1，而在此题里，则会返回0.

　　3. 若下一个字符不是数字，则返回0. 完全不考虑小数点和自然数的情况，不过这样也好，起码省事了不少。

　　4. 如果下一个字符是数字，则转为整形存下来，若接下来再有非数字出现，则返回目前的结果。

　　5. 还需要考虑边界问题，如果超过了整形数的范围，则用边界值替代当前值。

　　边界问题：INT_MAX = 2 147 483 647，INT_MIN = -2 147 483 648，如果当前数 res > INT_MAX / 10，即214 748 364，则下一个数字不管是多少，连在后面肯定越界；如果 res == INT_MAX / 10，则下一个数字只要不超过7，就不会越界。

public class Solution {
    public int myAtoi(String str) {
        if (str.isEmpty()) return 0;
        
        int n = str.length();
        int res = 0;
        int sign = 1;
        int i = 0;
        while (i < n && str.chatAt(i) == ‘ ‘) i++;
        if ((str.charAt(i) == ‘+‘) || (str.charAt(i) == ‘-‘))
            sign = str.charAt(i++) == ‘+‘ ? 1 : 0;
        while ((i < n) && (str.chatAt(i) >= ‘0‘) && (str.chatAt(i) <= ‘9‘)) {
            if (res > Integer.MAX_VALUE / 10 || (res == Integer.MAX_VALUE / 10 && str.chatAt(i) - ‘0‘ > 7))
                return sign == 1 ? Integer.MAX_VALUE : Integer.MIN_VALUE;
            res = res * 10 + (str.charAt(i++) - ‘0‘);
        }
        return res * sign;
    }
}