/********************************

Validate if a given string is numeric.

Some examples:

"0" => true

" 0.1 " => true

"abc" => false

"1 a" => false

"2e10" => true

Note: It is intended for the problem statement to be ambiguous. You should gather all requirements up front before implementing one.

**********************************/

class Solution {

public:

bool isNumber(string s) {

int n = s.size();

int i = 0;

bool flag = false;

/*

s1. leading whitespaces

s2. plus/minus sign;

s3. numbers.

s4. optional trailing whitespaces.

*/

//skip all white spaces;

while(i<n && s[i]==' ')i++;

if(i<n && (s[i] == '+' || s[i] == '-')) i++;//skip optional +/- characters

while(i<n && (s[i]>='0' && s[i]<='9')) //numbers.

{

flag = true;

i++;

}

if(i<n && s[i] == '.')

{

i++;

while(i<n && (s[i]>='0' && s[i]<='9')){i++;flag = true;}

}

if(flag == true && i<n && (s[i] == 'e'||s[i] == 'E'))

{

i++;

flag = false;//reset the flag needed

if(i<n && (s[i] == '+' || s[i] == '-')) i++;//skip optional +/- characters

while(i<n && (s[i]>='0' && s[i]<='9')){i++;flag = true;}

}

while(flag == true && i<n && s[i]==' ')i++;//skiping the trailing whitespaces

return flag&&i==n?true:false;

}

};