HDU 1754 I Hate It

题目链接：HDU 1754 I Hate It

线段树 单点更新 基础题目

刚开始学，感觉挺难。

用cin会超时。

#include <iostream>
#include <stdio.h>
#include <cstring>

using namespace std;

const int MAX_N = 200000 + 100;
const int MAX_M = 4 * MAX_N;
int n,m,res;
int a[MAX_N];

struct Node
{
    int left,right,_max;
};
Node node[MAX_M];

void build(int l,int r,int i)
{
    node[i].left = l;
    node[i].right = r;
    if(node[i].left == node[i].right)
    {
        node[i]._max = a[l];
        return;
    }
    int mid = (l + r) >> 1;
    build(l,mid,i << 1);
    build(mid + 1,r,i << 1 | 1);
    node[i]._max = max(node[i << 1]._max,node[i << 1 | 1]._max);
}
void update(int A,int B,int i)
{
    if(node[i].left == A && node[i].right == A)
    {
        node[i]._max = B;
        return;
    }
    int mid = (node[i].left + node[i].right) >> 1;
    if(A <= mid)
        update(A,B,i << 1);
    else
        update(A,B,i << 1 | 1);
    node[i]._max = max(node[i << 1]._max,node[i << 1 | 1]._max);
}
void query(int l,int r,int i)
{
    if(node[i].left == l && node[i].right == r)
    {
        res = max(res,node[i]._max);
        return;
    }
    int mid = (node[i].left + node[i].right) >> 1;
    if(mid < l)
        query(l,r,i << 1 | 1);
    else if(mid >= r)
        query(l,r,i << 1);
    else
    {
        query(mid + 1,r,i << 1 | 1);
        query(l,mid,i << 1);
    }
}
int main()
{
    //while(cin >> n >> m)
    while(scanf("%d%d",&n,&m) != EOF)
    {
        memset(a,0,sizeof(a));
        for(int i = 1;i <= n;i++)
            //cin >> a[i];
            scanf("%d",&a[i]);
        build(1,n,1);
        char c;
        int A,B;
        for(int i = 0;i < m;i++)
        {
           // cin >> c >> A >> B;
           scanf("%s%d%d",&c,&A,&B);
            if(c == 'Q')
            {
                res = 0;
                query(A,B,1);
                //cout << res << endl;
                printf("%d\n",res);
            }
            else if(c == 'U')
                update(A,B,1);
        }
    }
    return 0;
}