题目要求

Given a non-empty string containing an out-of-order English representation of digits 0-9, output the digits in ascending order.

Note:
Input contains only lowercase English letters.
Input is guaranteed to be valid and can be transformed to its original digits. That means invalid inputs such as "abc" or "zerone" are not permitted.

Input length is less than 50,000.

Example 1:
Input: "owoztneoer"
Output: "012"

Example 2:
Input: "fviefuro"
Output: "45"

一个非空的英文字符串，其中包含着乱序的阿拉伯数字的英文单词。如012对应的英文表达为zeroonetwo并继续乱序成owoztneoer。要求输入乱序的英文表达式，找出其中包含的所有0-9的数字，并按照从小到大输出。

思路和代码

首先将数字和英文表示列出来：

0 zero
1 one
2 two
3 three
4 four
5 five
6 six
7 seven
8 eight
9 nine

粗略一看，我们知道有许多字母只在一个英文数字中出现，比如z只出现在zero中。因此对于这种字母，它一旦出现，就意味着该数字一定出现了。
因此一轮过滤后可以得出只出现一次的字母如下：

0 zero -> z
1 one
2 two -> w
3 three
4 four -> u
5 five
6 six -> x
7 seven
8 eight
9 nine

再对剩下的数字字母过滤出只出现一次的字母：

1 one 
3 three -> r
5 five -> f
7 seven -> s
8 eight -> g
9 nine

最后对one和nine分别用o和i进行区分即可。因此可以得出如下代码：

    public String originalDigits(String s) {
        int[] letterCount = new int[26];
        for(char c : s.toCharArray()) {
            letterCount[c-'a']++;
        }
        
        int[] result = new int[10];
        
        //zero
        if((result[2] = letterCount['z'-'a']) != 0) {
            result[0] = letterCount['z' - 'a'];
            letterCount['z'-'a'] = 0;
            letterCount['e'-'a'] -= result[0];
            letterCount['r'-'a'] -= result[0];
            letterCount['o'-'a'] -= result[0];
        }
        //two
        if((result[2] = letterCount['w'-'a']) != 0) {
            letterCount['t'-'a'] -= result[2];
            letterCount['w'-'a'] = 0;
            letterCount['o'-'a'] -= result[2];
        }
        //four
        if((result[4] = letterCount['u'-'a']) != 0) {
            letterCount['f'-'a'] -= result[4];
            letterCount['o'-'a'] -= result[4];
            letterCount['u'-'a'] -= result[4];
            letterCount['r'-'a'] -= result[4];
        }
        //five
        if((result[5] = letterCount['f'-'a']) != 0) {
            letterCount['f'-'a'] -= result[5];
            letterCount['i'-'a'] -= result[5];
            letterCount['v'-'a'] -= result[5];
            letterCount['e'-'a'] -= result[5];
        }
        //six
        if((result[6] = letterCount['x'-'a']) != 0) {
            letterCount['s'-'a'] -= result[6];
            letterCount['i'-'a'] -= result[6];
            letterCount['x'-'a'] -= result[6];
        }
        //seven
        if((result[7] = letterCount['s'-'a']) != 0) {
            letterCount['s'-'a'] -= result[7];
            letterCount['e'-'a'] -= result[7] * 2;
            letterCount['v'-'a'] -= result[7];
            letterCount['n'-'a'] -= result[7];
        }
        //one
        if((result[1] = letterCount['o'-'a']) != 0) {
            letterCount['o'-'a'] -= result[1];
            letterCount['n'-'a'] -= result[1];
            letterCount['e'-'a'] -= result[1];
        }
        //eight
        if((result[8] = letterCount['g'-'a']) != 0) {
            letterCount['e'-'a'] -= result[8];
            letterCount['i'-'a'] -= result[8];
            letterCount['g'-'a'] -= result[8];
            letterCount['h'-'a'] -= result[8];
            letterCount['t'-'a'] -= result[8];
        }
        //nine
        if((result[9] = letterCount['i'-'a']) != 0) {
            letterCount['n'-'a'] -= result[9] * 2;
            letterCount['i'-'a'] -= result[9];
            letterCount['e'-'a'] -= result[9];
        }
        result[3] = letterCount['t'-'a'];
        StringBuilder sb = new StringBuilder();
        for(int i = 0 ; i<result.length ; i++) {
            for(int j = 0 ; j<result[i] ; j++) {
                sb.append(i);
            }
        }
        return sb.toString();
    }

上面的代码未免写的太繁琐了，对其进一步优化可以得到如下代码：

    public String originalDigits2(String s) {
        int[] alphabets = new int[26];
        for (char ch : s.toCharArray()) {
            alphabets[ch - 'a'] += 1;
        }
        
        int[] digits = new int[10];
        
        digits[0] = alphabets['z' - 'a'];
        digits[2] = alphabets['w' - 'a'];
        digits[6] = alphabets['x' - 'a'];
        digits[8] = alphabets['g' - 'a'];
        digits[7] = alphabets['s' - 'a'] - digits[6];
        digits[5] = alphabets['v' - 'a'] - digits[7];
        digits[3] = alphabets['h' - 'a'] - digits[8];
        digits[4] = alphabets['f' - 'a'] - digits[5];
        digits[9] = alphabets['i' - 'a'] - digits[6] - digits[8] - digits[5];
        digits[1] = alphabets['o' - 'a'] - digits[0] - digits[2] - digits[4];
        
        StringBuilder sb = new StringBuilder();
        for (int d = 0; d < 10; d++) {
            for (int count = 0; count < digits[d]; count++) sb.append(d);
        }
        
        return sb.toString();
    }