Re: [閒聊] 每日leetcode

[dont (dont)]

1930. Unique Length-3 Palindromic Subsequences

## 思路

先掃一次記錄每個字元 第一次跟最後一次出現的index
然後再看範圍內有幾種字元

## Code

```cpp
class Solution {
public:
    int countPalindromicSubsequence(string s) {
        vector<int> first(26, -1);
        vector<int> last(26, -1);

        int n = s.size();
        for (int i=0; i<n; ++i) {
            if (first[s[i]-'a'] == -1)
                first[s[i]-'a'] = i;
            last[s[i]-'a'] = i;
        }

        int res = 0;
        for (int i=0; i<26; ++i) {
            if (first[i] == -1)
                continue;
            bitset<26> seen = 0;
            for (int j = first[i] + 1; j < last[i]; ++j)
                seen[s[j]-'a'] = 1;
            res += seen.count();
        }

        return res;
    }
};
```

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※ 編輯: dont (194.195.89.211 日本), 01/04/2025 12:14:50