hdu 1754 线段树（单点替换 区间最值）

Sample Input
5 6
1 2 3 4 5
Q 1 5 //1-5结点的最大值
U 3 6 //将点3的数值换成6
Q 3 4
Q 4 5
U 2 9
Q 1 5

Sample Output
5
6
5
9

# include <iostream>
# include <cstdio>
# include <cstring>
# include <algorithm>
# include <cmath>
# include <queue>
# define LL long long
using namespace std ;

const int maxn = 200010;

int MAX[maxn<<2] ; //结点开4倍

void PushUP(int rt) //更新到父节点
{
    MAX[rt] = max(MAX[rt * 2] , MAX[rt * 2 + 1] ); //rt 为当前结点
}

void build(int l , int r , int rt) //构建线段树
{
    if (l == r)
    {
        scanf("%d" , &MAX[rt]) ;
        return ;
    }
    int m = (l + r) / 2 ;
    build(l , m , rt * 2) ;
    build(m + 1 , r , rt * 2 +1) ;
    PushUP(rt) ;
}

void updata(int p , int add , int l , int r , int rt)  //单点替换
{
    if (l == r)
    {
        MAX[rt] = add ;
        return ;
    }
    int m = (l + r) / 2 ;
    if (p <= m)
       updata(p , add , l , m , rt * 2) ;
    else
       updata(p , add , m + 1 , r , rt * 2 + 1) ;
    PushUP(rt) ;
}

int query(int L , int R , int l , int r , int rt)  //区间求最大值
{
    if (L <= l && r <= R)
        return MAX[rt] ;
    int m = (l + r) / 2 ;
    int ret = 0 ;
    if (L <= m)
       ret = max(ret , query(L , R , l , m , rt * 2) ) ;
    if (R > m)
       ret = max(ret , query(L , R , m + 1 , r , rt * 2 + 1) );
    return ret ;
}

int main ()
{
    //freopen("in.txt","r",stdin) ;
    int n , m ;
    while(scanf("%d %d" , &n , &m) != EOF)
    {
        build(1 , n , 1) ;
        char op[10] ;
        while(m--)
        {
            scanf("%s" , op) ;
            if (op[0] == 'E') //结束
               break ;
            int a , b ;
            scanf("%d %d" , &a , &b) ;
            if (op[0] == 'Q') //求a,b区间的和最大值
               printf("%d
", query(a , b , 1 , n , 1))  ;
            else if (op[0] == 'U')  //将第a个地方的值改为b
               updata(a , b , 1 , n , 1) ;
        }
    }

    return 0 ;
}