称号

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.

方法

分下面几个步骤： 

1. 将字符串的最前面和最后面的空格去掉。

2. 推断是否有e。并依据有e的情况进行推断：e前面是浮点。e后面必须是整形。

3. 没有e，则推断整个字符串是不是浮点。注意字符串前面的正负号别忘了。

PS：能够将使用小数点和e将字符串划分成3部分，进行推断。仅仅须要遍历一遍字符串。

能够參考：

private boolean isDouble(String s) {    	if (s.charAt(0) == '-' || s.charAt(0) == '+') {    		if (s.length() == 1) {    			return false;    		} else {    			s = s.substring(1);    		}    	}        int len = s.length();        boolean hasPoint = false;        boolean pointNext = false;        boolean pointPre = false;        for(int i = 0; i < len; i++) {            char cur = s.charAt(i);            if (cur == '.') {                if (hasPoint) {                    return false;                } else {                    hasPoint = true;                }            } else if (cur>= '0' && cur <= '9') {                if (hasPoint) {                    pointNext = true;                } else {                    pointPre = true;                }            } else {                return false;            }        }        if (hasPoint && (pointNext == false && pointPre == false)) {            return false;        } else {            return true;        }    }            private boolean isInteger(String s) {    	if (s.charAt(0) == '-' || s.charAt(0) == '+') {    		if (s.length() == 1) {    			return false;    		} else {    			s = s.substring(1);    		}    	}        int len = s.length();        for (int i = 0; i < len; i++) {            char cur = s.charAt(i);            if (!(cur>= '0' && cur <= '9')) {                return false;            }        }        return true;    }            public boolean isNumber(String s) {        if (s == null || s.length() == 0) {            return false;        }                 //        while(!s.equals("") && s.charAt(0) == ' ') {            s = s.substring(1);        }        while(!s.equals("") && s.charAt(s.length() - 1) == ' ') {            s = s.substring(0, s.length() - 1);        }        if (s.equals("")) {            return false;        }                int len = s.length();        for (int i = 0; i < len; i++) {            char cur = s.charAt(i);            if (cur == 'e') {                if ((i == 0 || i == len - 1)) {                    return false;                } else {                     return isDouble(s.substring(0, i)) && isInteger(s.substring(i + 1));                }            }         }        return isDouble(s);    }