Leetcode---Valid Number

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.

判断一个string是不是数字。非常麻烦的一道题，对于算法意义不大。

enum State {I, // integer                   ->{'+','-'}[0-9]+
D, // double                    ->Integer{'.'}[0-9]+
E, // number with exponential   ->{D,I}{'e','E'}Integer+
PI,// potential I               ->{'+','-'}
PD,// potential D               ->Integer{'.'}
PE,// potential E               ->{D,I}{'e','E'}{'+','-'}*
F  // Fail
};
bool allow(char c){
if('0' <= c && c <= '9')
return true;
char token[] = {'e', 'E', '.', '+', '-'};
for(unsigned int i = 0; i < sizeof(token)/sizeof(char); ++i){
if (c == token[i])
return true;
}
return false;
}

bool isNumber(const char *s) {
if(s == NULL)
return false;
while(*s == ' ')
s++;
string str(s);
while(str.back() == ' ')
str.pop_back();
if(str.length() == 0)
return false;
bool firstDigit = false;
for(unsigned int i = 0; i < str.length(); ++i){
char c = str[i];
if(!allow(c))
return false;
if(!firstDigit && '0' <= c && c <= '9')
firstDigit = true;
}
if(!firstDigit)
return false;
// str.length > 0, all characters are allowed, at least one digit
State state;
firstDigit = false;
// initialize state
switch(str[0]){
case '+':
case '-':
state = PI;
break;
case '.':
state = PD;
break;
case 'e':
case 'E':
state = F;
break;
default: // [0-9]
state = I;
firstDigit = true;
}

for(unsigned int i = 1; i < str.length() && state != F; ++i){
char c = str[i];
if(c == '+' || c == '-'){
switch(state){
case PE:
{
char prevChar = str[i-1];
if(prevChar != '+' || prevChar != '-')
state = PE;
else
state = F;
break;
}
default:
state = F;
}
}else if(c == 'e' || c == 'E'){
switch(state){
case I:
case D:
state = PE;
break;
case PD:
if(firstDigit)
state = PE;
else
state = F;
break;
default:
state = F;
}
}else if(c == '.'){
switch(state){
case I:
case PI:
state = PD;
break;
default:
state = F;
}
}else{ // c = [0-9]
firstDigit = true;
switch(state){
case I:
case PI:
state = I;
break;
case D:
case PD:
state = D;
break;
case E:
case PE:
state = E;
}
}
}

return state == I || state == D || state == PD || state == E;
}
};