HDU-1754 I hate it (裸线段树)

代码：

#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;
}