【一次过】Lintcode 1214. License Key Formatting

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You are given a license key represented as a string S which consists only alphanumeric character and dashes. The string is separated into N+1 groups by N dashes.

Given a number K, we would want to reformat the strings such that each group contains exactly K characters, except for the first group which could be shorter than K, but still must contain at least one character. Furthermore, there must be a dash inserted between two groups and all lowercase letters should be converted to uppercase.

Given a non-empty string S and a number K, format the string according to the rules described above.

样例

Input: S = "5F3Z-2e-9-w", K = 4

Output: "5F3Z-2E9W"

Explanation: The string S has been split into two parts, each part has 4 characters.
Note that the two extra dashes are not needed and can be removed.

Input: S = "2-5g-3-J", K = 2

Output: "2-5G-3J"

Explanation: The string S has been split into three parts, each part has 2 characters except the first part as it could be shorter as mentioned above.

注意事项

The length of string S will not exceed 12,000, and K is a positive integer.
String S consists only of alphanumerical characters (a-z and/or A-Z and/or 0-9) and dashes(-).
String S is non-empty.

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解题思路：

先将字符串S中的"-"用""替换，然后转换大写字母。从后向前遍历字符串，每遇到K个字符则添加"-"，即可。

public class Solution {
    /**
     * @param S: a string
     * @param K: a integer
     * @return: return a string
     */
    public String licenseKeyFormatting(String S, int K) {
        // write your code here
        String s = S.replace("-","").toUpperCase();
        
        StringBuilder res = new StringBuilder();
        int k = K;
        
        for(int i=s.length()-1 ; i>=0 ; ){
            if(k>0){
               res.insert(0,s.charAt(i--)); 
               k--;
            }else{
                res.insert(0,'-');
                k = K;
            }
        }
        
        return res.toString();
    }
}