# Valid Number

Like String To Integer, this is a problem need you take care of many cases carefully. We uses state transition or Finite State Machine to solve this problem.

public class ValidNumber {

static final int PREFIX = 0;
static final int PRE_POWER = 1;
static final int PRE_DECIMAL = 2;
static final int SIGN = 3;
static final int POWER_SIGN = 4;

static final int INTEGER = 5;
static final int DECIMAL = 6;
static final int POWER = 7;
static final int SUFFIX = 8;

public boolean isNumber(String s) {
int state = PREFIX;
for (int i = 0; i < s.length(); i++) {
char ch = s.charAt(i);

if (state == PREFIX) {
if (ch == ' ') continue;
else if (ch == '-' || ch == '+') state = SIGN;
else if (ch >= '0' && ch <= '9') state = INTEGER;
else if (ch == '.') state = PRE_DECIMAL;
else return false;
} else if (state == SIGN) {
if (ch >= '0' && ch <= '9') state = INTEGER;
else if (ch == '.') state = PRE_DECIMAL;
else return false;
} else if (state == PRE_DECIMAL) {
if (ch >= '0' && ch <= '9') state = DECIMAL;
else return false;
} else if (state == PRE_POWER) {
if (ch >= '0' && ch <= '9') state = POWER;
else if (ch == '+' || ch == '-') state = POWER_SIGN;
else return false;
} else if (state == POWER_SIGN) {
if (ch >= '0' && ch <= '9') state = POWER;
else return false;
} else if (state == INTEGER) {
if (ch >= '0' && ch <= '9') continue;
else if (ch == '.') state = DECIMAL;
else if (ch == 'e' || ch == 'E') state = PRE_POWER;
else if (ch == ' ') state = SUFFIX;
else return false;
} else if (state == DECIMAL) {
if (ch >= '0' && ch <= '9') continue;
else if (ch == 'e' || ch == 'E') state = PRE_POWER;
else if (ch == ' ') state = SUFFIX;
else return false;
} else if (state == POWER) {
if (ch >= '0' && ch <= '9') continue;
else if (ch == ' ') state = SUFFIX;
else return false;
} else if (state == SUFFIX) {
if (ch == ' ') continue;
else return false;
}
}

return (state >= INTEGER);
}
}