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https://leetcode.com/problems/excel-sheet-column-title/

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// because String is an immutable type, 
// using StringBuilder -> 0ms; using String -> 7ms.
class Solution_1 {
    public String convertToTitle(int n) {
        StringBuilder res=new StringBuilder();
        while(n>0){
            n--;
            res.insert(0,(char)('A'+n%26));
            n/=26;
        }
        return res.toString();
    }
}

// recursion method (keep rewriting the immutable String type)
class Solution_2 {
    public String convertToTitle(int n) {
        return n == 0 ? "" : convertToTitle(--n / 26) + (char)('A' + n % 26);
    }
}
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