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1859. Change Minimum Characters to Satisfy One of Three Conditions

My accepted C++ solution to LeetCode problem 1859, Change Minimum Characters to Satisfy One of Three Conditions, running in 9ms.

  • Difficulty: Medium
  • C++
  • Runtime 9ms
  • Memory 17.6MB

Read the problem on LeetCode View on GitHub

The problem statement is LeetCode’s and stays on their site. What follows is my accepted solution.

C++

Accepted on LeetCode — runtime 9ms, memory 17.6MB, accepted 2025-12-27.

cpp
class Solution {
    public:
    int minCharacters(string a, string b) {
        int m = a.size(), n = b.size();
        vector<int> ca(26), cb(26);
        for (char c : a) ca[c - 'a']++;
        for (char c : b) cb[c - 'a']++;

        int res = m + n;
        // Condition 3: make both strings same character
        for (int i = 0; i < 26; i++) {
            res = min(res, m + n - ca[i] - cb[i]);
        }

        // Conditions 1 & 2: prefix sum approach
        int preA = 0, preB = 0;
        for (int i = 0; i < 25; i++) {
            preA += ca[i];
            preB += cb[i];
            // Condition 1: all a < all b, so a has chars [0,i], b has chars [i+1,25]
            res = min(res, (m - preA) + preB);
            // Condition 2: all b < all a
            res = min(res, (n - preB) + preA);
        }
        return res;
    }
};

Source