HDU 1754 I Hate It(线段树求区间最大值)

#include<iostream>
#include<stdio.h>
#include<math.h>
#include<stdlib.h>
#include<algorithm>
#define N 200005
using namespace std;
int max(int a, int b)
{
    return a>b?a:b;
}
int val[N];
int maxn[N<<2];
void build(int l, int r, int rt)
{
    if(l==r)
    {
        scanf("%d", &val[l]);
//        printf("%d:%d ", l, val[l]);
        maxn[rt]=val[l];
        return ;
    }
    int mid=(l+r)/2;
    build(l, mid, rt<<1);
    build(mid+1, r, rt<<1|1);
    maxn[rt]=max(maxn[rt<<1], maxn[rt<<1|1]);
//    cout << maxn[rt] <<" " <<maxn[rt<<1]<<" "<< maxn[rt<<1|1] <<" ";
}

void update(int l, int r, int pos, int v, int rt)
{
    if(l==r)
    {
        maxn[rt]=v;
        return;
    }
    int mid=(l+r)/2;
    if(pos<=mid) update(l, mid, pos, v, rt<<1);
    else update(mid+1, r, pos, v, rt<<1|1);
    maxn[rt]=max(maxn[rt<<1], maxn[rt<<1|1]);
//    cout << maxn[rt<<1] << " " << maxn[rt<<1|1] << endl;
}

int query(int L, int R, int l, int r, int rt)
{
    if(L<=l&&r<=R)
    {
        return maxn[rt];
    }
    int mid=(l+r)/2;
    int s1=-1, s2=-1;
    if(L<=mid) s1= query(L, R, l, mid, rt<<1);
    if(R>mid)  s2= query(L, R, mid+1, r, rt<<1|1);
    return max(s1, s2);
}

int main()
{
    int n, m;
    char s[2];
    int a, b;
    while(scanf("%d%d", &n,&m)!=EOF)
    {
        build(1, n, 1);
        for(int i=0; i<m; i++)
        {
            scanf("%s", &s);
            if(s[0]=='Q')
            {
                scanf("%d%d", &a, &b);
                printf("%d\n",query(a, b, 1, n, 1));
            }
            else if(s[0]=='U')
            {
                scanf("%d%d", &a, &b);
                update(1, n, a, b, 1);
            }
        }
    }
    return 0;
}