第 355 场 LeetCode 周赛

A 按分隔符拆分字符串

简单模拟

class Solution {
public:
    vector<string> splitWordsBySeparator(vector<string> &words, char separator) {
        vector<string> res;
        for (auto &s: words) {
            int n = s.size();
            for (int i = 0, j = 0; i < n;) {
                while (j < n && s[j] != separator)
                    j++;
                if (i < j)
                    res.push_back(s.substr(i, j - i));
                i = j + 1;
                j = i;
            }
        }
        return res;
    }
};

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B 合并后数组中的最大元素

倒序遍历数组, 计算当前最右元素可以合并生成的最大元素

class Solution {
public:
    typedef long long ll;

    long long maxArrayValue(vector<int> &nums) {
        int n = nums.size();
        ll cur = nums[n - 1];//生成的当前元素
        ll res = nums[n - 1];
        for (int i = n - 2; i >= 0; i--) {
            if (nums[i] <= cur) {
                cur += nums[i];
            } else {//以nums[i]为新的最右元素
                cur = nums[i];
            }
            res = max(res, cur);
        }
        return res;
    }
};

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C 长度递增组的最大数目

先放个赛事凑出来的二分占位(不知道会不会被rejudge)，后续看看能不能补个证明或贴个其他方法…

class Solution {
public:
    typedef long long ll;

    int maxIncreasingGroups(vector<int> &usageLimits) {
        sort(usageLimits.begin(), usageLimits.end(), greater<>());
        int n = usageLimits.size();
        int l = 1, r = n;
        while (l < r) {
            int mid = (l + r + 1) / 2;
            ll cur = 0;
            for (int i = 0; i < n; i++) {
                if (i + 1 <= mid)
                    cur = min(1LL * (i + 1) * (mid * 2 - i) / 2, cur + usageLimits[i]);
                else
                    cur += usageLimits[i];
            }
            if (cur >= 1LL * mid * (mid + 1) / 2)
                l = mid;
            else
                r = mid - 1;
        }
        return l;
    }
};

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D 树中可以形成回文的路径数

dfs+计数: 设$u,v$两点之间路径上各字符数量的奇偶组成状态为$mask$, $u$和根节点之间路径上各字符数量的奇偶组成状态为$mas{k}_u$, $v$和根节点之间路径上各字符数量的奇偶组成状态为$mas{k}_v$, 则有$mask=mas{k}_u\wedge mas{k}_v$. 设$u,v$两点之间路径上各字符重排可以构成回文, 则 $mask$二进制表示最多有一位为1. 通过$dfs$用哈希记录各点与根节点路径的字符奇偶状态$mas{k}_k$, 然后枚举哈希中的$mas{k}_i$计算.

class Solution {
public:
    long long countPalindromePaths(vector<int> &parent, string s) {
        int n = parent.size();
        vector<pair<int, char>> sub[n];//邻接表
        for (int i = 1; i < n; i++)//建树
            sub[parent[i]].emplace_back(i, s[i]);
        unordered_map<int, int> cnt;//记录状态出现次数
        function<void(int, int)> dfs = [&](int cur, int mask) {
            cnt[mask]++;
            for (auto [j, ch]: sub[cur])
                dfs(j, mask ^ (1 << (ch - 'a')));
        };
        dfs(0, 0);
        long long res = 0;
        for (auto [mi, ci]: cnt) {
            if (ci > 1)//状态相同的情况
                res += 1LL * ci * (ci - 1);
            for (int j = 1; j < (1 << 26); j <<= 1)//枚举二进制只差一位的状态
                if (cnt.count(mi ^ j))
                    res += 1LL * ci * cnt[mi ^ j];
        }
        return res / 2;
    }
};