HDU1754 I Hate It

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字数统计: 548 | 阅读时长 ≈ 3

题面：http://acm.hdu.edu.cn/showproblem.php?pid=1754

题意

区间最大值查询，单点修改问题。线段树&分块等模板题

解法

分块或者线段树

分块例程

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#include<bits/stdc++.h>
using namespace std;
const int maxm = 2e3 + 10;
const int maxn = 2e5 + 10;
int l[maxm], r[maxm], b[maxn];
int a[maxn], belong[maxn];
int query(int x, int y)
{
    int res = 0;
    for(int i = x; i <= min(y, r[belong[x]]); i++)
        res = max(res, a[i]);
    if(belong[x] == belong[y])
        return res;
    for(int i = belong[x] + 1; i < belong[y]; i++)
        res = max(res, b[i]);
    for(int i = l[belong[y]]; i <= y; i++)
        res = max(res, a[i]);
    return res;
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n, m;
    while(cin >> n >> m)
    {
        memset(b, 0, sizeof(b));
        int block = sqrt(n);
        int num = n / block;
        if(n % block)num++;
        for(int i = 1; i <= n; i++)
            belong[i] = (i - 1) / block + 1;
        for(int i = 1; i <= num; i++)
        {
            l[i] = (i - 1) * block + 1;
            r[i] = i * block;//处理每块的左右边界;
        }
        r[num] = n;
        for(int i = 1; i <= n; i++)
        {
            cin >> a[i];
            b[belong[i]] = max(b[belong[i]], a[i]);
        }
        while(m--)
        {
            char c; int x, y;
            cin >> c >> x >> y;
            if(c == 'Q')
                cout << query(x, y) << endl;
            else{
                a[x] = y;
                b[belong[x]] = max(b[belong[x]], y);
            }
        }
    }
}

线段树例程

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#include<bits/stdc++.h>
using namespace std;
const int maxm = 5e3 + 10;
const int maxn = 1e6 + 10;
int a[maxn];
int n, dat[2 * maxn - 1];
void init(int N)
{
    n = 1;
    while(n < N)
        n <<= 1;
    for(int i = 0; i < 2 * n - 1; i++)
        dat[i] = INT_MIN;
}
void update(int k, int a)
{
    k += n - 1;
    dat[k] = a;
    while(k > 0)
    {
        k = (k - 1) / 2;
        dat[k] = max(dat[k * 2 + 1], dat[k * 2 + 2]);
    }
}
int query(int a, int b, int l, int r, int k)
{
    if(a >= r || b <= l)
        return INT_MIN;
    if(a <= l && b >= r)
        return dat[k];
    return max(query(a, b, l, (l + r) / 2, k * 2 + 1), query(a, b, (l + r) / 2, r, k * 2 + 2));
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    int N, m;
    string c;
    while(cin >> N >> m)
    {
        init(N);
        for(int i = 0; i < N; i++)
        {
            int t; cin >> t;
            update(i, t);
        }
        while(m--)
        {
            string q; int l, r;
            cin >> q >> l >> r;
            if(q[0] == 'Q')
                cout << query(l - 1, r, 0, n, 0) << endl;
            else
                update(l - 1, r);
        }
        
    }
    return 0;
}