1B - Spreadsheet was extremely challenging for me; it took me over 2 hours to get AC. I didn't find the editorial helpful, and then when I went to see practice solutions from red/orange people, it made even less sense (they were basically copy/paste with minor variation):

    #include<cstdio>
    void g(int t){if(t){g((t-1)/26);putchar(65+(t-1)%26);}}
    int main(){
    	int n,x,y;
    	char s[64],*p;
    	for(scanf("%d ",&n);n--;){
    		gets(s);
    		if(sscanf(s,"%*c%d%*c%d",&x,&y)==2){
    			g(y);
    			printf("%d\n",x);
    		}else{
    			for(x=0,p=s;*p>64;++p)
    				x=x*26+*p-64;
    			printf("R%sC%d\n",p,x);
    		}
    	}
    	return 0;
    }

I'll talk about each part of the problem itself and approaches/alternatives, and then explain how the above solution (henceforth "the solution") handles it. I'll go into unnecessary detail since I'm very new to CP/CF and would've wanted this level of detail myself.

Problem statement

Input: First line is an int representing the number of test cases. Each following line is a string of characters specifying a spreadsheet cell, in one of two formats:

• "Excel": ^[A-Z]+[0-9]+\$ (e.g. BB352).
• "RC": ^R[0-9]+C[0-9]+\$ (e.g. R253C55).

Output: For test case, we must output the same cell specified in the opposite format. Example:

2
R23C55
BC23

should output

BC23
R23C55

Constraints:

• Between 1 and 10000 test cases (all valid in either format)
• In each case, the row and column numbers are no larger than 1000000 (so each line is 16 chars at longest since we never exceed R1000000C1000000 / BDWGN1000000).

Approach

Our implementation needs to do 3 things:

• Convert from Excel to RC
• Convert from RC to Excel
• Determine which format the input is in, and do the right conversion on it

The heart of this problem is a change of base; RC-format uses two base-10 numbers, whereas Excel uses one for rows and a quasi-base-26 character string for columns.

Determine format

Note that for RC format, we will always find a C after some numbers in the string, whereas with Excel we will never find any letters after numbers. We could use this as a test of format with something like:

// assumes test case line in std::string in_str
bool found_num = false, is_excel = true;
for (char c: in_str){
    if ('0' <= c && c <= '9') {
        found_num = true;
    } else {
        is_excel = !found_num;
    }
}
// ...

The solution does this in a better way:

int n, x, y;
if (sscanf(s, "%*c%d%*c%d", &x, &y) == 2) {
  // RC to Excel
  ...
} else {
  // Excel to RC
  ...
}

The string %*c%d%*c%d will match on the RC format, but not Excel; %*c matches on and throws away any non-numeric characters, whereas %d matches on numeric characters and stores them in the respective variable. For an RC string, R and C are thrown away but the numbers following them are stored in x and y (so (sscanf() will return 2). On an Excel string, this won't match so nothing is stored in x or y (and sscanf() will return 0).

Excel to RC conversion

This is the easier conversion, conceptually. We can copy the row portion and print it as-is. For the column portion, we have to convert from base-26 to base-10. A base-10 number represents addition of successive powers of 10, each times a constant: 12345 in base-10 means 5*(10^0) + 4*(10^1) + 3*(10^2) + 2*(10^3) + 1*(10^4). So for the column string, we can map each letter to a number (i.e. A=1, B=2, ... , Z=26) and then associate each with a power of 26. Consider RZA: A=1, Z=26, and R=18, so we can write it as 1*(26^0) + 26*(26^1) + 18*(26^2) = 12845.

We have a problem: C++ has no built-in for integer exponentiation. There are a couple workarounds. We could try writing one ourselves:

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