8. String to Integer (atoi)

Implement the myAtoi(string s) function, which converts a string to a 32-bit signed integer.

The algorithm for myAtoi(string s) is as follows:

1. Whitespace: Ignore any leading whitespace (" ").
2. Signedness: Determine the sign by checking if the next character is '-' or '+', assuming positivity is neither present.
3. Conversion: Read the integer by skipping leading zeros until a non-digit character is encountered or the end of the string is reached. If no digits were read, then the result is 0.
4. Rounding: If the integer is out of the 32-bit signed integer range [-231, 231 - 1], then round the integer to remain in the range. Specifically, integers less than -231 should be rounded to -231, and integers greater than 231 - 1 should be rounded to 231 - 1.

Return the integer as the final result.

Example 1:

Input: s = "42"

Output: 42

Explanation:

The underlined characters are what is read in and the caret is the current reader position. Step 1: "42" (no characters read because there is no leading whitespace) ^ Step 2: "42" (no characters read because there is neither a '-' nor '+') ^ Step 3: "42" ("42" is read in) ^

Example 2:

Input: s = " -042"

Output: -42

Explanation:

Step 1: " -042" (leading whitespace is read and ignored) ^ Step 2: " -042" ('-' is read, so the result should be negative) ^ Step 3: " -042" ("042" is read in, leading zeros ignored in the result) ^

Example 3:

Input: s = "1337c0d3"

Output: 1337

Explanation:

Step 1: "1337c0d3" (no characters read because there is no leading whitespace) ^ Step 2: "1337c0d3" (no characters read because there is neither a '-' nor '+') ^ Step 3: "1337c0d3" ("1337" is read in; reading stops because the next character is a non-digit) ^

Example 4:

Input: s = "0-1"

Output: 0

Explanation:

Step 1: "0-1" (no characters read because there is no leading whitespace) ^ Step 2: "0-1" (no characters read because there is neither a '-' nor '+') ^ Step 3: "0-1" ("0" is read in; reading stops because the next character is a non-digit) ^

Example 5:

Input: s = "words and 987"

Output: 0

Explanation:

Reading stops at the first non-digit character 'w'.

Constraints:

• 0 <= s.length <= 200
• s consists of English letters (lower-case and upper-case), digits (0-9), ' ', '+', '-', and '.'.

class Solution {
public:
    int myAtoi(string s) {
        // 紀錄正負號，預設是正號
        int sign = 1;
        int res = 0;
        int i = 0;
        int n = s.size();
        // 掃 s，略過空白格
        while (i < n && s[i] == ' ') i++;

        // 紀錄正負號，i++
        if (i < n && s[i] == '+') {
            sign = 1;
            i++;
        } else if (i < n && s[i] == '-') {
            sign = -1;
            i++;
        }
        // 現在的 i 就是整數的起始位置
        while (i < n && isdigit(s[i])) {
            int digit = s[i] - '0';
            // 檢查有沒有超過 32-bit signed integer 的範圍
            if ((res > INT_MAX / 10) || (res == INT_MAX / 10 && digit > INT_MAX % 10)) {
                return sign == 1 ? INT_MAX : INT_MIN;
            }
            // 10 進位
            res = 10 * res + digit;
            i++;
        }
        return sign * res;
    }
};

• T: $O(N)$
• S: $O(1)$