HDU 1754 线段树

线段树单点更新

这道题需要注意的是输入，题目明确提示了要用scanf来输入加快速度，所以这里可能会有一个错误，在build里面是用scanf输入的。而输入文件可能是在输入N个数据后有一个换行符，如果不把换行符读出来可能会有错误。所以要用下面这句话scanf("%*c%c%d %d", &c, &a, &b);

#include<iostream>
#include<string>
#include<fstream>
using namespace std;
int N, M;
#define maxn 200010
#define Lchild rt<<1,L,m
#define Rchild rt<<1|1,m+1,R
#define MAX(a,b) a>b?a:b
int SegmentTree[maxn * 3];
void push_up(int rt)
{
SegmentTree[rt] = MAX(SegmentTree[rt << 1], SegmentTree[rt << 1 | 1]);
}
void build(int rt=1,int L=1,int R=N)
{
if (L == R)
{
//cin >> SegmentTree[rt];
scanf_s("%d", &SegmentTree[rt]);
return;
}
int m = (L + R) >> 1;
build(Lchild);
build(Rchild);
push_up(rt);
}
void update(int p, int delta, int rt = 1, int L = 1, int R = N)
{
if (L == R)
{
SegmentTree[rt] = delta;
return;
}
int m = (L + R) >> 1;
if (p <= m)
update(p, delta, Lchild);
else
update(p, delta, Rchild);
push_up(rt);
}
int query(int l, int r, int rt = 1, int L = 1, int R = N)
{
if (l <= L&&R <= r)
{
return SegmentTree[rt];
}
int m = (L + R) >> 1;
int ret = -1;
if (l <= m)
{
int temp = query(l, r, Lchild);
ret = MAX(ret, temp);
}
if (r > m)
{
int te = query(l, r, Rchild);
ret = MAX(ret,te);
}
return ret;
}
int main()
{
while (cin>>N>>M)
{
build();
char c;
int a, b;
for (int i = 1; i <= M; i++)
{
//cin >> c >> a >> b;
scanf("%*c%c%d %d", &c, &a, &b);

if (c == 'Q')
{
cout << query(a, b) << endl;
}
else if (c == 'U')
{
update(a, b);
}
}
}
}