String to Integer (atoi)

Convert the beginning of the string s into a 32-bit signed integer by applying these steps in order:

1. Spaces: skip any leading space characters (' '). Only the space character counts as whitespace.
2. Sign: if the next character is '+' or '-', read it as the sign. At most one sign character is read; with no sign the number is positive.
3. Digits: read consecutive digits until you reach a non-digit character or the end of the string. Leading zeros are allowed and add nothing. Everything after the digits is ignored.
4. No digits: if step 3 read no digits, return 0.
5. Clamp: apply the sign. If the result is below -2^31 (-2147483648), return -2147483648; if it is above 2^31 - 1 (2147483647), return 2147483647.

Any other character met before the digits (a letter, a ., a second sign, or a space after the sign) stops parsing at that point.