HDU 1754 I Hate It 线段树

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
int mx;
int s1[200100],s2[1000000];
int build(int l,int r,int p)
{
   if (l==r) return s2[p]=s1[l];
   int m=(l+r)/2;
   int a=build(l,m,2*p);
   int b=build(m+1,r,2*p+1);
   return s2[p]=max(a,b);
}
int un(int l,int r,int p,int pos,int c)
{
    if (l==r&&l==pos) return s2[p]=c;
    if (l>pos||r<pos) return s2[p];
    int m=(l+r)/2;
    int a=un(l,m,2*p,pos,c);
    int b=un(m+1,r,2*p+1,pos,c);
    return s2[p]=max(a,b);
}
void find(int l,int r,int p,int ll,int rr)
{
    if (ll<=l&&rr>=r) {mx=max(mx,s2[p]);return ;}
    if (l>rr||r<ll) return ;
 // if (l==r) return ;
    int m=(l+r)/2;
    find(l,m,2*p,ll,rr);
    find(m+1,r,2*p+1,ll,rr);
    return ;
}
int main()
{
    int n,m,i,a,b;
    char s;
    while (~scanf("%d%d",&n,&m))
    {
        for (i=1;i<=n;i++) scanf("%d",&s1[i]);
        build(1,n,1);
        while (m--)
        {
            getchar();
            scanf("%c %d %d",&s,&a,&b);
           // printf("****%c****%d***%d\n",s,a,b);
            if (s==‘U‘) un(1,n,1,a,b);
            else
            {
                mx=0;
                int ra=min(a,b);
                int rb=max(a,b);
                find(1,n,1,ra,rb);
                printf("%d\n",mx);
            }
        }
    }
    return 0;
}