Question

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Excel中的行列数用A~Z 26个字母表示，A, B, C, D, …, Z, AA, AB, …, AZ, BA, BB, … 分别表示10进制数1, 2, 3, 4, …, 26, 27, 28, …, 52, 53, 54…。

请实现2个函数decToExcel和excelToDec，将10进制数转换为Excel数，以及将Excel数转换为10进制数。

Solution

Note the indexing starts from 1, not from 0. This caused some trouble for me.

Code

written by me

public String decToExcel(int decNum) {
    decNum--;
    int digits = 1;
    int exponen = 26;
    while (decNum >= exponen) {
        decNum -= exponen;
        exponen *= 26;
        digits++;
    }
    // now we know the total number of digits
    int num = decNum;
    int total = exponen / 26;
    String ans = "";
    for (int i = 0; i < digits; i++) {
        ans += (char) (num / total + 'A');
        num %= total;
        total /= 26;
    }
    return ans;
}

public int excelToDec(String excelNum) {
    int digits = excelNum.length();
    int total = 1;
    int sum = 1;
    for (int i = 1; i < digits; i++) {
        total *= 26;
        sum += total;
    }
    for (int i = 0; i < digits; i++) {
        sum += (excelNum.charAt(i) - 'A') * total;
        total /= 26;
    }
    return sum;
}

updated code: we can actually do it recursively. The code is much more concise.

//将十进制数转换为excel数
public String decToExcel(int decNum) {
    if (decNum == 0) {
        return "";
    }
    decNum--;
    char last = (char) ('A' + decNum % 26);
    return decToExcel(decNum / 26) + last;
}

//将excel数转换为十进制数
public int excelToDec(String excelNum) {
    if (excelNum.equals("")) {
        return 0;
    }
    int len = excelNum.length();
    int last = 1 + excelNum.charAt(len - 1) - 'A';
    return excelToDec(excelNum.substring(0, len - 1)) * 26 + last;
}