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Add Solution.java to problems 0091

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Original file line number	Diff line number	Diff line change
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	`1`	`+class Solution {`
	`2`	`+ public int numDecodings(String s) {`
	`3`	`+ if (s.charAt(0) == '0') return 0;`
	`4`	`+`
	`5`	`+ char[] chars = s.toCharArray();`
	`6`	`+ return decode(chars, chars.length - 1);`
	`7`	`+ }`
	`8`	`+`
	`9`	`+ int decode(char[] chars, int index) {`
	`10`	`+ if (index <= 0) return 1;`
	`11`	`+`
	`12`	`+ int count = 0;`
	`13`	`+ char curr = chars[index];`
	`14`	`+ char prev = chars[index - 1];`
	`15`	`+`
	`16`	`+ // 当前字符比 "0" 大,则直接利用它之前的字符串所求得的结果`
	`17`	`+ if (curr > '0') {`
	`18`	`+ count = decode(chars, index - 1);`
	`19`	`+ }`
	`20`	`+ // 由前一个字符和当前字符所构成的数字,值必须要在 1 到 26 之间,否则无法进行解码`
	`21`	`+ if (prev == '1' \|\| (prev == '2' && curr <= '6')) {`
	`22`	`+ count += decode(chars, index - 2);`
	`23`	`+ }`
	`24`	`+ return count;`
	`25`	`+ }`
	`26`	`+}`

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