HDU-1754 I hate it (裸線段樹)

代碼：

#include <stdio.h>
#include <string.h>

#include <iostream>
#include <algorithm>

using namespace std;

struct node
{
    int maxn;
    int begin;
    int end;
} tree[880000];

int n, m;
int pa[220000];

int build(int root, int begin, int end)
{
    tree[root].begin = begin;
    tree[root].end = end;
    if (begin == end) {
        scanf("%d", &tree[root].maxn);
        pa[begin] = root;
        return tree[root].maxn;
    } else {
        int left = build(root << 1, begin, begin+(end-begin)/2);
        int right = build(root << 1 | 1, begin+(end-begin)/2+1, end);
        tree[root].maxn = max(left, right);
        return tree[root].maxn;
    }
}

void update(int root)
{
    int left = tree[root<<1].maxn;
    int right = tree[root<<1|1].maxn;
    tree[root].maxn = max(left, right);
    if (root != 1) {
        update(root >> 1);
    }
}

int query(int root, int front, int rear)
{
    if (front == tree[root].begin && rear == tree[root].end) {
        return tree[root].maxn;
    } else if (front > rear) {
        return 0;
    }

    int middle = tree[root].begin + (tree[root].end - tree[root].begin) / 2;

    if (rear < middle + 1) {
        return query(root << 1, front, rear);
    } else if (front > middle) {
        return query(root << 1 | 1, front, rear);
    } else {
        int left = query(root << 1, front, middle);
        int right = query(root << 1 | 1, middle+1, rear);
        return max(left, right);
    }
}

int main()
{
    int x, y;
    char command[110];

    while (scanf("%d%d", &n, &m) != EOF) {
        build(1, 1, n);
        for (int i = 1; i <= m; i++) {
            scanf("%s%d%d", command, &x, &y);
            if (command[0] == 'Q') {
                int maxn = query(1, x, y);
                printf("%d\n", maxn);
            } else if (command[0] == 'U') {
                tree[pa[x]].maxn = y;
                update(pa[x]>>1);
            }
        }
    }

    return 0;
}