8. String to Integer (atoi)

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8. String to Integer (atoi)

Description

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:

  • Whitespace : Ignore any leading whitespace (" ").
  • Signedness : Determine the sign by checking if the next character is '-' or '+', assuming positivity if neither present.
  • Conversion : Read the integer by skipping leading zerosuntil a non-digit character is encountered or the end of the string is reached. If no digits were read, then the result is 0.
  • Rounding : If the integer is out of the 32-bit signed integer range [-2^31, 2^31 - 1], then round the integer to remain in the range. Specifically, integers less than -2^31 should be rounded to -2^31, and integers greater than 2^31 - 1 should be rounded to 2^31 - 1.

Return the integer as the final result.

Example 1:

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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:

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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:

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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:

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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:

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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 '.'.

Hints/Notes

Solution

Language: C++

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class Solution {
public:
    int myAtoi(string s) {
        int i = 0, n = s.size();
        while (i < n && s[i] == ' ') {
            i++;
        }
        if (i == n) return 0;
        int sign = 1;
        if (s[i] == '-') {
            sign = -1;
            i++;
        } else if (s[i] == '+') {
            i++;
        }
        long num = 0;
        while (i < n && isdigit(s[i])) {
            int digit = s[i] - '0';
            num = num * 10 + digit;
            if (num * sign > INT_MAX) {
                return INT_MAX;
            }
            if (num * sign < INT_MIN) {
                return INT_MIN;
            }
            i++;
        }
        return num * sign;
    }
};