Convert String to Integer
Question
Convert the inputted string to an integer. For example, “345” will output 345.
Solution
Though it looks simple, in fact it is pretty tricky. First we need to make the concept clear.

Scan each character from the left to right, then multiply the previously obtained integer by ten before adding the scanned digit to it.

Integer can be positive or negative, check if the first character is
+
or
. 
Check for exceptions such as nondigit or null input.

Check for overflow. Overflow means the inputted integer is greater than the maximum size of integer that can be stored in memory. In this case we should output 0, or throw an exception.
string_to_integer(string)
if string is null then return 0
positive_int = true
long_number = 0
for i equals 0 to length of string
if i == 0
if string[i] == '+' then continue
else if string[i] == '' then positive_int = false
continue
if string[i] >= 0 and string[i] <= 9 then
digit = string[i]  '0'
long_number = long_number * 10 + digit
if int_overflow(long_number) then
long_number = 0
throw exception
else
long_number = 0
throw exception
if not positive_int
then long_number = 0  long_number
return long_number
Example (Python)
import sys
def int_overflow(long):
return not sys.maxint1 <= long <= sys.maxint
def string_to_integer(string):
if string is None: return 0
positive_int = True
long_number = 0
for i in xrange(0, len(string)):
print "checking: " + string[i]
if i == 0:
if string[i] == '+': continue
elif string[i] == '': positive_int = False
continue
if ord('0') <= ord(string[i]) <= ord('9'):
digit = ord(string[i])  ord('0')
long_number = long_number * 10 + digit
if int_overflow(long_number):
long_number = 0
return Exception('integer overflew')
else:
long_number = 0
return Exception('invalid digit in string')
if not positive_int:
long_number = 0  long_number
return long_number
print "result: {}".format(string_to_integer('345'))
Example (C++)
enum Status {
kValid = 0, kInvalid
};
int g_nStatus = kValid;
int StrToInt(const char * str) {
g_nStatus = kInvalid;
long long num = 0;
if (str != NULL) {
const char * digit = str;
// the first char in the string maybe '+' or ''
bool minus = false;
if ( * digit == '+') digit++;
else if ( * digit == '') {
digit++;
minus = true;
}
// the remaining chars in the string
while ( * digit != '/0') {
if ( * digit >= '0' && * digit <= '9') {
num = num * 10 + ( * digit  '0'); // overflow
if (num > std::numeric_limits < int > ::max()) {
num = 0;
break;
}
digit++;
} else {
// if the char is not a digit, invalid input
num = 0;
break;
}
}
if ( * digit == '/0') {
g_nStatus = kValid;
if (minus) num = 0  num;
}
}
return static_cast<int>(num);
}
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