package main;

public class String2Long {

public long stringToLong(String s) {

if (null == s) {

throw new IllegalArgumentException("Null string.");

}

int len = s.length();

boolean isNagtive = false;

if(0 == len) {

throw new IllegalArgumentException("Empty string.");

}

//Get the first character and check the sign

if(s.charAt(0) == '-')

{

s = s.substring(1);

isNagtive = true;

}

else if(s.charAt(0) == '+')

{

s = s.substring(1);

}

//try to convert the rest of string to long

//define result as long to make sure it won't overflow

long result = 0;

for (int i = 0; i < s.length(); i++)

{

int value = s.charAt(i) - '0';

//check the invalid character

if(value < 0 || value > 9)

throw new NumberFormatException("Invalid string.");

result *= 10;

//check the Overflow case

if(result > Long.MAX_VALUE)

throw new NumberFormatException("Number Overflow.");

result += value;

}

//covert the result to long based on sign

return (isNagtive ? -1 : 1) * (long) result;

}

}