第 372 场 LeetCode 周赛题解

A 使三个字符串相等

求三个串的最长公共前缀

class Solution {
public:
    int findMinimumOperations(string s1, string s2, string s3) {
        int n1 = s1.size(), n2 = s2.size(), n3 = s3.size();
        int i = 0;
        for (; i < min({n1, n2, n3}); i++)
            if (!(s1[i] == s2[i] && s2[i] == s3[i]))
                break;
        if (i == 0)
            return -1;
        return n1 + n2 + n3 - i * 3;
    }
};

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B 区分黑球与白球

双指针：一个指针 $i$ 遍历字符串，一个指针 $j$ 指向下一个 $0$ 应该移动到位置，$i$ 指向 $0$ 时更新答案同时 $j+1$

class Solution {
public:
    long long minimumSteps(string s) {
        long long res = 0;
        for (int j = 0, i = 0; i < s.size(); i++)
            if (s[i] == '0') {
                res += i - j;
                j++;
            }
        return res;
    }
};

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C 最大异或乘积

贪心：从高到低枚举二进制每一位 $i\in [0,n)$，有两种情况：
1）$a$ 和 $b$ 这一位相同，则存在 $x$ 使得 $a\wedge x$ 和 $b\wedge x$ 这一位都为 $1$ ;
2) $a$ 和 $b$ 这一位不同，则$a\wedge x$ 和 $b\wedge x$ 中只有一个数这一位为 $1$ ，若当前 $a\wedge x$ 不考虑这一位的数大于$b\wedge x$ 不考虑这一位的数时，这一位 $1$ 应该给 $b\wedge x$ ，否则给 $a\wedge x$

class Solution {
public:
    using ll = long long;

    int maximumXorProduct(long long a, long long b, int n) {
        ll mod = 1e9 + 7;
        ll na = a, nb = b;
        for (ll i = n - 1; i >= 0; i--)
            if ((na >> i & 1LL) == (nb >> i & 1LL)) {
                na |= 1LL << i;
                nb |= 1LL << i;
            } else {
                if ((na & (~(1LL << i))) > (nb & (~(1LL << i)))) {
                    nb |= 1LL << i;
                    na &= ~(1LL << i);
                } else {
                    na |= 1LL << i;
                    nb &= ~(1LL << i);
                }
            }
        return (na % mod * (nb % mod) % mod + mod) % mod;
    }
};

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D 找到 Alice 和 Bob 可以相遇的建筑

二分+线段树：对于一个查询 $[a,b]$ $(a\le b)$ ， 有三种情况：
1）$a==b$，答案为 $a$
2）$a\ne b$ ,且 $heights[a]<heights[b]$ ，答案为 $b$
3）$a\ne b$ ,且 $heights[a]\ge heights[b]$，答案为满足 $max\{heights[k]|k\in [b+1,i]\}>heights[a]$ 的最小的 $i$ ，通过线段树来维护区间最大值，然后通过二分求 $i$

class SegmentTree {
public:
    typedef long long ll;

    inline void push_down(ll index) {
        st[index << 1].lazy = 1;
        st[index << 1 | 1].lazy = 1;
        st[index << 1].mark = max(st[index << 1].mark, st[index].mark);
        st[index << 1 | 1].mark = max(st[index << 1 | 1].mark, st[index].mark);
        st[index << 1].s = max(st[index << 1].s, st[index].mark);
        st[index << 1 | 1].s = max(st[index << 1 | 1].s, st[index].mark);
        st[index].lazy = 0;
    }

    inline void push_up(ll index) {
        st[index].s = max(st[index << 1].s, st[index << 1 | 1].s);
    }

    SegmentTree(vector<int> &init_list) {
        st = vector<SegmentTreeNode>(init_list.size() * 4 + 10);
        build(init_list, 1, init_list.size());
    }

    void build(vector<int> &init_list, ll l, ll r, ll index = 1) {
        st[index].tl = l;
        st[index].tr = r;
        st[index].lazy = 0;
        st[index].mark = 0;
        if (l == r) {
            st[index].s = init_list[l - 1];
        } else {
            ll mid = (l + r) >> 1;
            build(init_list, l, mid, index << 1);
            build(init_list, mid + 1, r, index << 1 | 1);
            push_up(index);
        }
    }

    ll query(ll l, ll r, ll index = 1) {
        if (l <= st[index].tl and st[index].tr <= r) {
            return st[index].s;
        } else {
            if (st[index].lazy)
                push_down(index);
            if (r <= st[index << 1].tr)
                return query(l, r, index << 1);
            else if (l > st[index << 1].tr)
                return query(l, r, index << 1 | 1);
            return max(query(l, r, index << 1), query(l, r, index << 1 | 1));
        }
    }

private:
    struct SegmentTreeNode {
        ll tl;
        ll tr;
        ll s;
        ll mark;
        int lazy;
    };
    vector<SegmentTreeNode> st;
};


class Solution {
public:
    vector<int> leftmostBuildingQueries(vector<int> &heights, vector<vector<int>> &queries) {
        int n = heights.size();
        SegmentTree stree(heights);
        vector<int> res;
        res.reserve(queries.size());
        for (auto &qi: queries) {
            int a = min(qi[0], qi[1]), b = max(qi[0], qi[1]);
            if (a == b)
                res.push_back(a);
            else if (heights[a] < heights[b])
                res.push_back(b);
            else {//ha>hb
                int l = b + 1, r = n;
                while (l < r) {
                    int mid = (l + r) / 2;
                    if (stree.query(b + 1 + 1, mid + 1) > heights[a])
                        r = mid;
                    else
                        l = mid + 1;
                }
                if (l < n)
                    res.push_back(l);
                else
                    res.push_back(-1);
            }
        }
        return res;
    }
};

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