2531. Make Number of Distinct Characters Equal 

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class Solution {
    public boolean isItPossible(String word1, String word2) {
        Map<Character, Integer> countMap1 = new HashMap<>();
        Map<Character, Integer> countMap2 = new HashMap<>();

        for (int i = 0; i < word1.length(); i++) {
            char c = word1.charAt(i);
            countMap1.merge(c, 1, Integer::sum);
        }
        for (int i = 0; i < word2.length(); i++) {
            char c = word2.charAt(i);
            countMap2.merge(c, 1, Integer::sum);
        }

        for (Map.Entry<Character, Integer> entry1 : countMap1.entrySet()) {
            char c1 = entry1.getKey();
            int count1 = entry1.getValue();
            for (Map.Entry<Character, Integer> entry2 : countMap2.entrySet()) {
                char c2 = entry2.getKey();
                int count2 = entry2.getValue();

                if (c1 == c2) {
                    // 两个字符相同，若两个字符串的不同字符数相等，则可交换 c1 与 c2
                    if (countMap1.size() == countMap2.size()) {
                        return true;
                    }
                } else {
                    // 两个字符不相同，此时尝试交换 c1 与 c2
                    if (countMap1.size() - (count1 == 1 ? 1 : 0) + (!countMap1.containsKey(c2) ? 1 : 0) ==
                            countMap2.size() - (count2 == 1 ? 1 : 0) + (!countMap2.containsKey(c1) ? 1 : 0)) {
                        return true;
                    }
                }
            }
        }

        return false;
    }
}

关键点在于比较交换后的两个字符串中的不同字符数是否相等。

References

2531. Make Number of Distinct Characters Equal