线段树求最值

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

代码：

#include<stdio.h>
#include<iostream>
#include<algorithm>
using namespace std;
#define maxn 200005
int  tree[4*maxn];
void build(int left,int right,int root)
{
    if(left==right)
    {
        scanf("%d",&tree[root]);
    }
    else
    {
        int mid=(left+right)/2;
        build(left,mid,root*2);
        build(mid+1,right,root*2+1);
        tree[root]=max(tree[root*2],tree[root*2+1]);
    }
}
void Pointupdate(int pos,int newn,int left,int right,int root)
{
    if(left==right)
    {
        tree[root]=newn;
    }
    else
    {
        int mid=(left+right)/2;
        if(pos<=mid)
            Pointupdate(pos,newn,left,mid,root*2);
        else
            Pointupdate(pos,newn,mid+1,right,root*2+1);
        tree[root]=max(tree[root*2],tree[root*2+1]);

    }
}
int query(int x,int y,int left,int right,int root)
{
    if(x<=left&&right<=y)
    {
        return tree[root];
    }
    else
    {
        int cnt=0;
        int mid=(left+right)/2;
        if(x<=mid)
          cnt=max(cnt,query(x,y,left,mid,root*2));
        if(y>mid)
          cnt=max(cnt,query(x,y,mid+1,right,root*2+1));
          return cnt;
    }

}
int main()
{
    //freopen("in.txt","r",stdin);
    int n,m;
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        build(1,n,1);
        while(m--)
        {
            char ch[3];
            int x,y;
            scanf("%s%d%d",ch,&x,&y);
            if(ch[0]=='Q')
                printf("%d\n",query(x,y,1,n,1));
            else
                Pointupdate(x,y,1,n,1);
        }
    }
    return 0;
}