String to Integer (atoi)

https://leetcode.com/problems/string-to-integer-atoi/description/

Implementatoito convert a string to an integer.

Hint:Carefully consider all possible input cases. If you want a challenge, please do not see below and ask yourself what are the possible input cases.

Notes:It is intended for this problem to be specified vaguely (ie, no given input specs). You are responsible to gather all the input requirements up front.

Thoughts

注意 1. 开头空白 2. 符号 3. 非数字的 4. 溢出.

Code

class Solution {
    public int myAtoi(String str) {
        int res = 0, sign = 1;
        // Empty String
        if (str == null || str.length() == 0) {
            return res;
        }

        int i = 0;

        // RM white spaces
        while (str.charAt(i) == ' ') {
            i++;
        }

        // Handle signs
        if (str.charAt(i) == '+' || str.charAt(i) == '-') {
            sign = str.charAt(i) == '+' ? 1 : -1;
            i++;
        }

        // Convert number and avoid overflow
        for (; i < str.length(); i++) {
            int digit = str.charAt(i) - '0';
            if (digit >= 0 && digit <= 9) {
                if (Integer.MAX_VALUE / 10 < res || Integer.MAX_VALUE / 10 == res && Integer.MAX_VALUE % 10 < digit) {
                    return sign == 1 ? Integer.MAX_VALUE : Integer.MIN_VALUE;
                }
                res = res * 10 + digit;
                //System.out.println(res + ", " + (res - digit) / 10);
            } else {
                break;
            }
        }

        return res * sign;
    }
}

Analysis

时间复杂度O(N), N为str长度.