2268. Minimum Number of Keypresses

You have a keypad with 9 buttons, numbered from 1 to 9, each mapped to lowercase English letters. You can choose which characters each button is matched to as long as:

• All 26 lowercase English letters are mapped to.
• Each character is mapped to by exactly 1 button.
• Each button maps to at most 3 characters.

To type the first character matched to a button, you press the button once. To type the second character, you press the button twice, and so on.

Given a string s, return the minimum number of keypresses needed to type s using your keypad.

Note that the characters mapped to by each button, and the order they are mapped in cannot be changed.

class Solution {
public:
    int minimumKeypresses(string s) {
        unordered_map<char, int> counter;
        for(auto& ch : s)
            counter[ch]++;
        vector<int> counts;
        for(auto& [_, c] : counter)
            counts.push_back(c);
        sort(rbegin(counts), rend(counts));
        int res = 0;
        for(int i = 0; i < counts.size(); i++) {
            res += counts[i] * ((i / 9) + 1);
        }
        return res;
    }
};